FastClasspathScanner

FastClasspathScanner is an uber-fast, ultra-lightweight classpath scanner for Java, Scala and other JVM languages.

(The main repo is here, in case this is a GitHub fork.)

What is classpath scanning? Classpath scanning involves scanning directories and jar/zip files on the classpath to find files (especially classfiles) that meet certain criteria. In many ways, classpath scanning offers the inverse of the Java reflection API:

The Java reflection API can tell you the superclass of a given class, but classpath scanning can find all classes that extend a given superclass.
The Java reflection API can give you the list of annotations on a given class, but classpath scanning can find all classes that are annotated with a given annotation.
etc. (Many other classpath scanning objectives are listed below.)

Classpath scanning can also be used to produce a visualization of the class graph (the "class hierarchy"). Class graph visualizations can be useful in understanding complex codebases, and for finding architectural design issues (e.g. in the graph visualization below, you can see that ShapeImpl only needs to implement Shape, not Renderable, because Renderable is already a superinterface of Shape). [see graph legend here]

FastClasspathScanner is able to:

find classes that subclass a given class or one of its subclasses;
find interfaces that extend a given interface or one of its subinterfaces;
find classes that implement an interface or one of its subinterfaces, or whose superclasses implement the interface or one of its subinterfaces;
find classes that have a specific class annotation or meta-annotation;
find the constant literal initializer value in a classfile's constant pool for a specified static final field;
find files (even non-classfiles) anywhere on the classpath that have a path that matches a given string or regular expression;
find all classes that contain a field of a given type (including identifying fields based on array element type and generic parameter type);
perform the actual classpath scan;
detect changes to the files within the classpath since the first time the classpath was scanned, or alternatively, calculate the MD5 hash of classfiles while scanning, in case using timestamps is insufficiently rigorous for change detection;
return a list of the names of all classes, interfaces and/or annotations on the classpath (after whitelist and blacklist filtering);
return a list of all directories and files on the classpath (i.e. all classpath elements) as a list of File objects, with the list deduplicated and filtered to include only classpath directories and files that actually exist, saving you from the complexities of working with the classpath and classloaders; and
generate a GraphViz .dot file from the class graph for visualization purposes, as shown above. A class graph visualizatoin depicts connections between classes, interfaces, annotations and meta-annotations, and connections between classes and the types of their fields.

Benefits of FastClasspathScanner compared to other classpath scanning methods:

FastClasspathScanner parses the classfile binary format directly, instead of using reflection, which makes scanning particularly fast. (Reflection causes the classloader to load each class, which can take an order of magnitude more time than parsing the classfile directly, and can lead to unexpected behavior due to static initializer blocks of classes being called on class load.)
FastClasspathScanner is extremely lightweight, as it does not depend on any classfile/bytecode parsing or manipulation libraries like Javassist or ObjectWeb ASM.
FastClasspathScanner handles many diverse and complicated means used to specify the classpath, and has a pluggable architecture for handling other classpath specification methods (in the general case, finding all classpath elements is not as simple as reading the java.class.path system property and/or getting the path URLs from the system URLClassLoader).
FastClasspathScanner has built-in support for generating GraphViz visualizations of the classgraph, as shown above.
FastClasspathScanner can find classes not just by annotation, but also by meta-annotation (e.g. if annotation A annotates annotation B, and annotation B annotates class C, you can find class C by scanning for classes annotated by annotation A). This makes annotations more powerful, as they can be used as a hierarchy of inherited traits (similar to how interfaces work in Java). In the graph above, the class Figure has the annotation @UIWidget, and the annotation class UIWidget has the annotation @UIElement, so by transitivity, Figure also has the meta-annotation @UIElement.
FastClasspathScanner can find all classes that have fields of a given type (this feature is normally only found in an IDE, e.g. References > Workspace in Eclipse).

Usage

There are two different mechanisms for using FastClasspathScanner. (The two mechanisms can be used together.)

Mechanism 1:

Create a FastClasspathScanner instance, passing the constructor a whitelist of package prefixes to scan within (and/or a blacklist of package prefixes to ignore);
Add one or more MatchProcessor instances to the FastClasspathScanner by calling a .match...() method on the FastClasspathScanner instance;
Optionally call .verbose() to give verbose output for debugging purposes; and
Call .scan() to start the scan.

This is the pattern shown in the following example. (Note: this example uses Java 8 lambda expressions to automatically construct the appropriate type of MatchProcessor corresponding to each .match...() method, based on a MatchProcessor being a FunctionalInterface. See the Tips section for the Java 7 equivalent.)

// Package prefixes to scan are listed in the constructor:
// -- "com.xyz.widget" is whitelisted for scanning;
// -- "com.xyz.widget.internal" is blacklisted (ignored), as it is prefixed by "-".
new FastClasspathScanner("com.xyz.widget", "-com.xyz.widget.internal")  
    .matchSubclassesOf(Widget.class,
        // c is a subclass of Widget or a descendant subclass.
        // This lambda expression is of type SubclassMatchProcessor.
        c -> System.out.println("Subclass of Widget: " + c.getName()))
    .matchSubinterfacesOf(Tweakable.class,
        // c is an interface that extends the interface Tweakable.
        // This lambda expression is of type SubinterfaceMatchProcessor.
        c -> System.out.println("Subinterface of Tweakable: " + c.getName()))
    .matchClassesImplementing(Changeable.class,
        // c is a class that implements the interface Changeable; more precisely,
        // c or one of its superclasses implements the interface Changeable, or
        // implements an interface that is a descendant of Changeable.
        // This lambda expression is of type InterfaceMatchProcessor.
        c -> System.out.println("Implements Changeable: " + c.getName()))
    .matchClassesWithAnnotation(BindTo.class,
        // c is a class annotated with BindTo.
        // This lambda expression is of type AnnotationMatchProcessor.
        c -> System.out.println("Has a BindTo class annotation: " + c.getName()))
    .matchStaticFinalFieldNames("com.xyz.widget.Widget.LOG_LEVEL",
        // The following method is called when any static final fields with
        // names matching one of the above fully-qualified names are
        // encountered, as long as those fields are initialized to constant
        // values. The value returned is the value in the classfile, not the
        // value that would be returned by reflection, so this can be useful
        // in hot-swapping of changes.
        // This lambda expression is of type StaticFinalFieldMatchProcessor.
        (String className, String fieldName, Object fieldConstantValue) ->
            System.out.println("Static field " + fieldName + " in class "
            + className + " " + " currently has constant literal value "
            + fieldConstantValue + " in the classfile"))
    .matchFilenamePattern("^template/.*\\.html",
        // relativePath is the section of the matching path relative to the
        // classpath element it is contained in; fileContentBytes is the content
        // of the file.
        // This lambda expression is of type FileMatchContentProcessor.
        (relativePath, fileContentBytes) ->
            registerTemplate(relativePath, new String(fileContentBytes, "UTF-8")))
    // Optional, in case you want to debug any issues with scanning
    .verbose()
    // Actually perform the scan
    .scan();

// [...Some time later...]
// See if any timestamps on the classpath are more recent than the time of the
// previous scan. Much faster than standard classpath scanning, because
// only timestamps are checked, and jarfiles don't have to be opened.
boolean classpathContentsModified =
    fastClassPathScanner.classpathContentsModifiedSinceScan();

Mechanism 2: Create a FastClasspathScanner instance, potentially without adding any MatchProcessors, then call scan() to scan the classpath, then call .getNamesOf...() methods to get a list of classes, interfaces and annotations of interest without actually calling the classloader on any matching classes.

The .getNamesOf...() methods return sorted lists of strings, rather than lists of Class<?> references, and scanning is done by reading the classfile directly, so the classloader does not need to be called for these methods to return their results. This can be useful if the static initializer code for matching classes would trigger unwanted side effects if run during a classpath scan. An example of this usage pattern is:

List<String> subclassesOfWidget = new FastClasspathScanner("com.xyz.widget")
    // No need to add any MatchProcessors, just create a new scanner and then call
    // .scan() to parse the class hierarchy of all classfiles on the classpath.
    .scan()
    // Get the names of all subclasses of Widget on the classpath,
    // again without calling the classloader:
    .getNamesOfSubclassesOf("com.xyz.widget.Widget");

Note that Mechanism 2 only works with class, interface and annotation matches; there are no corresponding .getNamesOf...() methods for filename pattern or static field matches, since these methods are only looking at the DAG of whitelisted classes and interfaces encountered during the scan.

Downloading

You can get a pre-built JAR (usable in JRE 1.7 or later) from Sonatype, or add the following Maven Central dependency:

<dependency>
    <groupId>io.github.lukehutch</groupId>
    <artifactId>fast-classpath-scanner</artifactId>
    <version>LATEST</version>
</dependency>

Tips

Use with Java 7: FastClasspathScanner needs to be built with JDK 8, since MatchProcessors are declared with a @FunctionalInterface annotation, which does not exist in JDK 7. (There are no other Java 8 features in use in FastClasspathScanner currently.) If you need to build with JDK 1.7, you can always manually remove the @FunctionalInterface annotations from the MatchProcessors. However, the project can be compiled in Java 7 compatibility mode, which does not complain about these annotations, and can generate a jarfile that works with both Java 7 and Java 8. The jarfile available from Maven Central is compatible with Java 7.

The usage examples shown above use lambda expressions (functional interfaces) from Java 8 in the Mechanism 1 examples for syntactic simplicity. The Java 7 equivalent is as follows (note that there is a different MatchProcessor class corresponding to each .match...() method, e.g. .matchSubclassesOf() takes a SubclassMatchProcessor):

new FastClasspathScanner("com.xyz.widget")  
    .matchSubclassesOf(Widget.class, new SubclassMatchProcessor<Widget>() {
        @Override
        public void processMatch(Class<? extends Widget> matchingClass) {
            System.out.println("Subclass of Widget: " + matchingClass))
        }
    })
    .scan();

Note that the first usage of Java 8 features like lambda expressions or Streams incurs a one-time startup penalty of 30-40ms (this startup cost is incurred by the JDK, not FastClasspathScanner).

Protip: using Java 8 method references: The .match...() methods (e.g. .matchSubclassesOf()) take a MatchProcessor as one of their arguments, which are single-method interfaces annotated with @FunctionalInterface. FunctionalInterfaces are interchangeable as long as the number and types of arguments match. You may find it useful to use Java 8 method references in the place of MatchProcessors, e.g. if you have a variable List<Class<? extends Widget>> matchingClasses, its .add() method can be referenced using matchingClasses::add, which has a single parameter of type Class<? extends Widget>. This method reference is interchangeable with SubclassMatchProcessor<T>::processMatch(Class<? extends T> matchingClass) assuming you call the .matchSubclassesOf() with Widget.class as the type parameter:

List<Class<? extends Widget>> matchingClasses = new ArrayList<>();
new FastClasspathScanner("com.xyz.widget")
    .matchSubclassesOf(Widget.class, matchingClasses::add)  // Method ref for List.add()
    .scan();

Classpath mechanisms handled by FastClasspathScanner

FastClasspathScanner handles a number of classpath specification mechanisms, including some non-standard ClassLoader implementations:

The java.class.path system property, supporting specification of the classpath using the -cp JRE commandline switch.
The standard Java URLClassLoader, and both standard and custom subclasses. (Some runtime environments override URLClassLoader for their own purposes, but do not set java.class.path -- FastClasspathScanner fetches classpath URLs from all visible URLClassLoaders.)
Class-Path references in a jarfile's META-INF/MANIFEST.MF, whereby jarfiles may add other external jarfiles to their own classpaths. FastClasspathScanner is able to determine the transitive closure of these references, breaking cycles if necessary.
The JBoss/WildFly custom classloader mechanism.
The WebLogic custom classloader mechanism.
Eventually, the Java 9 module system [work has not started on this yet -- patches are welcome].

[Note that if you have a custom classloader in your runtime that is not covered by one of the above cases, you can add your own ClassLoaderHandler, which will be loaded from your own project's jarfile by FastClasspathScanner using the Java ServiceLoader framework, via an entry in META-INF/services.]

API

Most of the methods in the API return this (of type FastClasspathScanner), so that you can use the method chaining calling style, as shown in the example above.

Note that the | character is used below to compactly describe overloaded methods below, e.g. getNamesOfSuperclassesOf(Class<?> subclass | String subclassName).

Constructor

You can pass a scanning specification to the constructor of FastClasspathScanner to describe what should be scanned. This prevents irrelevant classpath entries from being unecessarily scanned, which can be time-consuming. (Note that calling the constructor does not start the scan, you must separately call .scan() to perform the actual scan.)

// Constructor for FastClasspathScanner
public FastClasspathScanner(String... scanSpec)

The constructor accepts a list of whitelisted package prefixes / jar names to scan, as well as blacklisted packages/jars not to scan, where blacklisted entries are prefixed with the '-' character. For example:

new FastClasspathScanner("com.x") limits scanning to the package com.x and its sub-packages in all jarfiles and all directory entries on the classpath.
new FastClasspathScanner("com.x", "-com.x.y") limits scanning to com.x and all sub-packages except com.x.y in all jars and directories on the classpath.
new FastClasspathScanner("com.x", "javax.persistence.Entity") limits scanning to com.x but also returns links between whitelisted classes and the non-whitelisted annotation class javax.persistence.Entity -- see below for more info.
new FastClasspathScanner("com.x", "-com.x.BadClass") scans within com.x, but blacklists the class com.x.BadClass. Note that a capital letter after the final '.' indicates a whitelisted or blacklisted class, as opposed to a package.
new FastClasspathScanner("com.x", "-com.x.y", "jar:deploy.jar") limits scanning to com.x and all its sub-packages except com.x.y, but only looks in jars named deploy.jar on the classpath. Note:
1. Whitelisting one or more jar entries prevents non-jar entries (directories) on the classpath from being scanned.
2. Only the leafname of a jarfile can be specified in a jar: or -jar: entry, so if there is a chance of conflict, make sure the jarfile's leaf name is unique.
new FastClasspathScanner("com.x", "-jar:irrelevant.jar") limits scanning to com.x and all sub-packages in all directories on the classpath, and in all jars except irrelevant.jar. (i.e. blacklisting a jarfile only excludes the specified jarfile, it doesn't prevent all directories from being scanned, as with whitelisting a jarfile.)
new FastClasspathScanner("com.x", "jar:") limits scanning to com.x and all sub-packages, but only looks in jarfiles on the classpath -- directories are not scanned. (i.e. "jar:" is a wildcard to indicate that all jars are whitelisted, and as in the example above, whitelisting jarfiles prevents non-jars (directories) from being scanned.)
new FastClasspathScanner("com.x", "-jar:") limits scanning to com.x and all sub-packages, but only looks in directories on the classpath -- jarfiles are not scanned. (i.e. "-jar:" is a wildcard to indicate that all jars are blacklisted.)
new FastClasspathScanner(): If you don't specify any whitelisted package prefixes, all jarfiles and all directories on the classpath will be scanned, with the exception of the java and sun packages, which are always blacklisted for efficiency (e.g. java.lang, java.util etc. are never scanned).

Notes on scan specs:

Scan spec entries can use either . or / as the package/directory separator, i.e. new FastClasspathScanner("com.x.y") and new FastClasspathScanner("com/x/y") are equivalent.
Superclasses, subclasses etc. that are in a package that is not whitelisted (or that is blacklisted) will not be returned by a query, but can be used to query. For example, consider a class com.external.X that is a superclass of com.xyz.X, with a whitelist scanSpec of com.xyz. Then .getNamesOfSuperclassesOf("com.xyz.X") will return an empty result, but .getNamesOfSubclassesOf("com.external.X") will return ["com.xyz.X"].
For efficiency, system, bootstrap and extension jarfiles like rt.jar (i.e. the jarfiles distributed with the JRE) are always blacklisted, as are package prefixes java and sun, i.e. they are never scanned. If you put custom classes into the lib/ext directory in your JRE folder (which is a valid but rare way of adding jarfiles to the classpath), they will be ignored by association with the JRE.

Detecting annotations, superclasses and implemented interfaces outside of whitelisted packages

In general, FashClasspathScanner cannot find relationships between classes, interfaces and annotations unless the entire path of references between them falls within a whitelisted (and non-blacklisted) package.

However, as shown below, it is possible to match based on references to "external" classes, defined as superclasses, implemented interfaces, superinterfaces and annotations/meta-annotations that are defined outside of the whitelisted packages but that are referred to by a class defined within a whitelisted package. (An external class is a class that is exactly one reference link away from a class in a whitelisted package.) External classes are not whitelisted by default, so are not returned by .getNamesOf...() methods.

ou may also whitelist an external class name in the scan spec passed to the constructor. This will cause the external class to be returned by .getNamesOf...() methods. The constructor determines that a passed string is a class name and not a package name if the letter after the last '.' is in upper case, as per standard Java conventions for package and class names.

// Given a class com.xyz.MyEntity that is annotated with javax.persistence.Entity:

// Result: ["com.xyz.MyEntity"], because com.xyz.MyEntity is in the whitelisted path com.xyz
List<String> matches1 = new FastClasspathScanner("com.xyz").scan()
    .getNamesOfClassesWithAnnotation("javax.persistence.Entity");

// Result: [], because javax.persistence.Entity is not explicitly whitelisted, and is not defined
// in a whitelisted package
List<String> matches2 = new FastClasspathScanner("com.xyz").scan()
    .getNamesOfAllAnnotationClasses();

// Result: ["javax.persistence.Entity"], because javax.persistence.Entity is explicitly whitelisted
List<String> matches3 = new FastClasspathScanner("com.xyz", "javax.persistence.Entity").scan()
    .getNamesOfAllAnnotationClasses();

1. Matching the subclasses (or finding the superclasses) of a class

FastClasspathScanner can find all classes on the classpath within whitelisted package prefixes that extend a given superclass.

Important note: the ability to detect that a class extends another depends upon the entire ancestral path between the two classes being within one of the whitelisted package prefixes. (However, see above for info on "external" class references.)

You can scan for classes that extend a specified superclass by calling .matchSubclassesOf() with a SubclassMatchProcessor parameter before calling .scan(). This method will call the classloader on each matching class (using Class.forName()) so that a class reference can be passed into the match processor. There are also methods List<String> getNamesOfSubclassesOf(String superclassName) and List<String> getNamesOfSubclassesOf(Class<?> superclass) that can be called after .scan() to find the names of the subclasses of a given class (whether or not a corresponding match processor was added to detect this) without calling the classloader.

Furthermore, the methods List<String> getNamesOfSuperclassesOf(String subclassName) and List<String> getNamesOfSuperclassesOf(Class<?> subclass) are able to return all superclasses of a given class after a call to .scan(). (Note that there is not currently a SuperclassMatchProcessor or .matchSuperclassesOf().)

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface SubclassMatchProcessor<T> {
    public void processMatch(Class<? extends T> matchingClass);
}

public <T> FastClasspathScanner matchSubclassesOf(Class<T> superclass,
    SubclassMatchProcessor<T> subclassMatchProcessor)

// Mechanism 2: Call one of the following after calling .scan():

public List<String> getNamesOfSubclassesOf(
    Class<?> superclass | String superclassName)

public List<String> getNamesOfSuperclassesOf(
    Class<?> subclass | String subclassName)

2. Matching the subinterfaces (or finding the superinterfaces) of an interface

FastClasspathScanner can find all interfaces on the classpath within whitelisted package prefixes that that extend a given interface or its subinterfaces.

Important note: The ability to detect that an interface extends another interface depends upon the entire ancestral path between the two interfaces being within one of the whitelisted package prefixes. (However, see above for info on "external" class references.)

You can scan for interfaces that extend a specified superinterface by calling .matchSubinterfacesOf() with a SubinterfaceMatchProcessor parameter before calling .scan(). This method will call the classloader on each matching class (using Class.forName()) so that a class reference can be passed into the match processor. There are also methods List<String> getNamesOfSubinterfacesOf(String ifaceName) and List<String> getNamesOfSubinterfacesOf(Class<?> iface) that can be called after .scan() to find the names of the subinterfaces of a given interface (whether or not a corresponding match processor was added to detect this) without calling the classloader.

Furthermore, the methods List<String> getNamesOfSuperinterfacesOf(String ifaceName) and List<String> getNamesOfSuperinterfacesOf(Class<?> iface) are able to return all superinterfaces of a given interface after a call to .scan(). (Note that there is not currently a SuperinterfaceMatchProcessor or .matchSuperinterfacesOf().)

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface SubinterfaceMatchProcessor<T> {
    public void processMatch(Class<? extends T> matchingInterface);
}

public <T> FastClasspathScanner matchSubinterfacesOf(Class<T> superInterface,
    SubinterfaceMatchProcessor<T> subinterfaceMatchProcessor)

// Mechanism 2: Call one of the following after calling .scan():

public List<String> getNamesOfSubinterfacesOf(
    Class<?> superinterface | String superinterfaceName)

public List<String> getNamesOfSuperinterfacesOf(
    Class<?> subinterface | String subinterfaceName)

3. Matching the classes that implement an interface

FastClasspathScanner can find all classes on the classpath within whitelisted package prefixes that that implement a given interface. The matching logic here is trickier than it would seem, because FastClassPathScanner also has to match classes whose superclasses implement the target interface, or classes that implement a sub-interface (descendant interface) of the target interface, or classes whose superclasses implement a sub-interface of the target interface.

Important note: The ability to detect that a class implements an interface depends upon the entire ancestral path between the class and the interface (and any relevant sub-interfaces or superclasses along the path between the two) being within one of the whitelisted package prefixes. (However, see above for info on "external" class references.)

You can scan for classes that implement a specified interface by calling .matchClassesImplementing() with a InterfaceMatchProcessor parameter before calling .scan(). This method will call the classloader on each matching class (using Class.forName()) so that a class reference can be passed into the match processor. There are also methods List<String> getNamesOfClassesImplementing(String ifaceName) and List<String> getNamesOfClassesImplementing(Class<?> iface) that can be called after .scan() to find the names of the classes implementing a given interface (whether or not a corresponding match processor was added to detect this) without calling the classloader.

N.B. There are also convenience methods for matching classes that implement all of a given list of annotations (an "and" operator).

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface InterfaceMatchProcessor<T> {
    public void processMatch(Class<? extends T> implementingClass);
}

public <T> FastClasspathScanner matchClassesImplementing(
    Class<T> implementedInterface,
    InterfaceMatchProcessor<T> interfaceMatchProcessor)

// Mechanism 2: Call one of the following after calling .scan():

public List<String> getNamesOfClassesImplementing(
    Class<?> implementedInterface | String implementedInterfaceName)

public List<String> getNamesOfClassesImplementingAllOf(
    Class<?>... implementedInterfaces | String... implementedInterfaceNames)

4. Matching classes with a specific annotation or meta-annotation

FastClassPathScanner can detect classes that have a specified annotation. This is the inverse of the Java reflection API: the Java reflection API allows you to find the annotations on a given class, but FastClasspathScanner allows you to find all classes that have a given annotation.

Important note: The ability to detect that an annotation annotates or meta-annotates a class depends upon the annotation and the class being within one of the whitelisted package prefixes. (However, see above for info on "external" class references.)

FastClassPathScanner also allows you to detect meta-annotations (annotations that annotate annotations that annotate a class of interest). Java's reflection methods (e.g. Class.getAnnotations()) do not directly return meta-annotations, they only look one level back up the annotation graph. FastClasspathScanner follows the annotation graph, allowing you to scan for both annotations and meta-annotations using the same API. This allows for the use of multi-level annotations as a means of implementing "multiple inheritance" of annotated traits. (Compare with @dblevins' metatypes.)

Consider this graph of classes (A, B and C) and annotations (D..L): [see graph legend here]

Class A is annotated by F and meta-annotated by J.
Class B is annonated or meta-annotated by all the depicted annotations except for G (since all annotations but G can be reached along a directed path of annotations from B)
Class C is only annotated by G.
Note that the annotation graph can contain cycles: here, H annotates I and I annotates H. These are handled appropriately by FastClasspathScanner by determining the transitive closure of the directed annotation graph.

You can scan for classes with a given annotation or meta-annotation by calling .matchClassesWithAnnotation() with a ClassAnnotationMatchProcessor parameter before calling .scan(), as shown below, or by calling .getNamesOfClassesWithAnnotation() or similar methods after calling .scan().

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface ClassAnnotationMatchProcessor {
    public void processMatch(Class<?> matchingClass);
}

public FastClasspathScanner matchClassesWithAnnotation(
    Class<?> annotation,
    ClassAnnotationMatchProcessor classAnnotationMatchProcessor)

// Mechanism 2: Call one of the following after calling .scan():

// (a) Get names of classes that have the specified annotation(s)
// or meta-annotation(s)

public List<String> getNamesOfClassesWithAnnotation(
    Class<?> annotation | String annotationName)

public List<String> getNamesOfClassesWithAnnotationsAllOf(
    Class<?>... annotations | String... annotationNames)

public List<String> getNamesOfClassesWithAnnotationsAnyOf(
    Class<?>... annotations | String... annotationNames)

// (b) Get names of annotations that have the specified meta-annotation

public List<String> getNamesOfAnnotationsWithMetaAnnotation(
    Class<?> metaAnnotation | String metaAnnotationName)

// (c) Get the annotations and meta-annotations on a class or interface,
// or the meta-annotations on an annotation. This is more powerful than
// Class.getAnnotations(), because it also returns meta-annotations.

public List<String> getNamesOfAnnotationsOnClass(
    Class<?> classOrInterface | String classOrInterfaceName)

public List<String> getNamesOfMetaAnnotationsOnAnnotation(
    Class<?> annotation | String annotationName)

Properties of the annotation scanning API:

There are convenience methods for matching classes that have AnyOf a given list of annotations/meta-annotations (an OR operator), and methods for matching classes that have AllOf a given list of annotations/meta-annotations (an AND operator).
The method getNamesOfClassesWithAnnotation() (which maps from an annotation/meta-annotation to classes it annotates/meta-annotates) is the inverse of the method getNamesOfAnnotationsOnClass() (which maps from a class to annotations/meta-annotations on the class; this is related to Class.getAnnotations() in the Java reflections API, but it returns not just direct annotations on a class, but also meta-annotations that are in the transitive closure of the annotation graph, starting at the class of interest).
The method getNamesOfAnnotationsWithMetaAnnotation() (which maps from meta-annotations to annotations they meta-annotate) is the inverse of the method getNamesOfMetaAnnotationsOnAnnotation() (which maps from annotations to the meta-annotations that annotate them; this also retuns the transitive closure of the annotation graph, starting at an annotation of interest).

5. Fetching the constant initializer values of static final fields

FastClassPathScanner is able to scan the classpath for matching fully-qualified static final fields, e.g. for the fully-qualified field name com.xyz.Config.POLL_INTERVAL, FastClassPathScanner will look in the class com.xyz.Config for the static final field POLL_INTERVAL, and if it is found, and if it has a constant literal initializer value, that value will be read directly from the classfile and passed into a provided StaticFinalFieldMatchProcessor.

Field values are obtained directly from the constant pool in a classfile, not from a loaded class using reflection. This allows you to detect changes to the classpath and then run another scan that picks up the new values of selected static constants without reloading the class. (Class reloading is fraught with issues.)

This can be useful in hot-swapping of changes to static constants in classfiles if the constant value is changed and the class is re-compiled while the code is running. (Neither the JVM nor the Eclipse debugger will hot-replace static constant initializer values if you change them while running code, so you can pick up changes this way instead).

Note: The visibility of the fields is not checked; the value of the field in the classfile is returned whether or not it should be visible to the caller. Therefore you should probably only use this method with public static final fields (not just static final fields) to coincide with Java's own semantics.

// Only Mechanism 1 is applicable -- attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface StaticFinalFieldMatchProcessor {
    public void processMatch(String className, String fieldName,
    Object fieldConstantValue);
}

public FastClasspathScanner matchStaticFinalFieldNames(
    HashSet<String> fullyQualifiedStaticFinalFieldNames
    | String fullyQualifiedStaticFinalFieldName
    | String[] fullyQualifiedStaticFinalFieldNames,
    StaticFinalFieldMatchProcessor staticFinalFieldMatchProcessor)

Note: Only static final fields with constant-valued literals are matched, not fields with initializer values that are the result of an expression or reference, except for cases where the compiler is able to simplify an expression into a single constant at compiletime, such as in the case of string concatenation. The following are examples of constant static final fields:

static final int w = 5;          // Literal ints, shorts, chars etc. are constant
static final String x = "a";     // Literal Strings are constant
static final String y = "a" + "b";  // Referentially equal to interned String "ab"
static final byte b = 0x7f;      // StaticFinalFieldMatchProcessor is passed boxed types (here Byte)
private static final int z = 1;  // Visibility is ignored; non-public constant fields also match

whereas the following fields are non-constant, non-static and/or non-final, so these fields cannot be matched:

static final Integer w = 5;         // Non-constant due to autoboxing
static final String y = "a" + w;    // Non-constant because w is non-constant
static final int[] arr = {1, 2, 3}; // Arrays are non-constant
static int n = 100;                 // Non-final
final int q = 5;                    // Non-static

Primitive types (int, long, short, float, double, boolean, char, byte) are boxed in the corresponding wrapper class (Integer, Long etc.) before being passed to the provided StaticFinalFieldMatchProcessor.

6. Finding files (even non-classfiles) anywhere on the classpath whose path matches a given string or regular expression

This can be useful for detecting changes to non-classfile resources on the classpath, for example a web server's template engine can hot-reload HTML templates when they change by including the template directory in the classpath and then detecting changes to files that are in the template directory and have the extension ".html".

A FileMatchProcessor is passed the InputStream for any File or ZipFileEntry in the classpath that has a path matching the pattern provided in the .matchFilenamePattern() method (or other related methods, see below). You do not need to close the passed InputStream if you choose to read the stream contents; the stream is closed by the caller.

The value of relativePath is relative to the classpath entry that contained the matching file.

// Only Mechanism 1 is applicable -- attach a MatchProcessor before calling .scan():

// Use this interface if you want to be passed an InputStream.  N.B. you do not
// need to close the InputStream before exiting, it is closed by the caller.
@FunctionalInterface
public interface FileMatchProcessor {
    public void processMatch(String relativePath, InputStream inputStream,
        int inputStreamLengthBytes) throws IOException;
}

// Use this interface if you want to be passed a byte array with the file contents.
@FunctionalInterface
public interface FileMatchContentsProcessor {
    public void processMatch(String relativePath, byte[] fileContents)
        throws IOException;
}

// The following two MatchProcessor variants are available if you need to know
// which classpath element (i.e. which directory or zipfile) that the match was
// found within. (File classpathElt is the root for relativePath.) 

@FunctionalInterface
public interface FileMatchProcessorWithContext {
    public void processMatch(File classpathElt, String relativePath,
        InputStream inputStream, int inputStreamLengthBytes) throws IOException;
}

@FunctionalInterface
public interface FileMatchContentsProcessorWithContext {
    public void processMatch(File classpathElt, String relativePath,
        byte[] fileContents) throws IOException;
}

// Pass one of the above FileMatchProcessor variants to one of the following methods:

// Match a pattern, such as "^com/pkg/.*\\.html$"
public FastClasspathScanner matchFilenamePattern(String pathRegexp,
        FileMatchProcessor fileMatchProcessor
        | FileMatchProcessorWithContext fileMatchProcessorWithContext 
        | FileMatchContentsProcessor fileMatchContentsProcessor
        | FileMatchContentsProcessorWithContext fileMatchContentsProcessorWithContext)
        
// Match a (non-regexp) relative path, such as "com/pkg/WidgetTemplate.html"
public FastClasspathScanner matchFilenamePath(String relativePathToMatch,
        FileMatchProcessor fileMatchProcessor
        | FileMatchProcessorWithContext fileMatchProcessorWithContext 
        | FileMatchContentsProcessor fileMatchContentsProcessor
        | FileMatchContentsProcessorWithContext fileMatchContentsProcessorWithContext)
        
// Match a leafname, such as "WidgetTemplate.html"
public FastClasspathScanner matchFilenameLeaf(String leafToMatch,
        FileMatchProcessor fileMatchProcessor
        | FileMatchProcessorWithContext fileMatchProcessorWithContext 
        | FileMatchContentsProcessor fileMatchContentsProcessor
        | FileMatchContentsProcessorWithContext fileMatchContentsProcessorWithContext)
        
// Match a file extension, e.g. "html" matches "WidgetTemplate.html"
public FastClasspathScanner matchFilenameExtension(String extensionToMatch,
        FileMatchProcessor fileMatchProcessor
        | FileMatchProcessorWithContext fileMatchProcessorWithContext 
        | FileMatchContentsProcessor fileMatchContentsProcessor
        | FileMatchContentsProcessorWithContext fileMatchContentsProcessorWithContext)

7. Find all classes that contain a field of a given type

One of the more unique capabilities of FastClasspathScanner is to find classes in the whitelisted (non-blacklisted) package hierarchy that have fields of a given type, assuming both the class and the types of its fields are in whitelisted (non-blacklisted) packages. (In particular, you cannot search for fields of a type defined in a system package, e.g. java.lang.String or java.lang.Object, because system packages are always blacklisted.)

Matching field types also matches type parameters and array types. For example, .getNamesOfClassesWithFieldOfType("com.xyz.Widget") will match classes that contain fields of the form:

Widget widget
Widget[] widgets
ArrayList<? extends Widget> widgetList
HashMap<String, Widget> idToWidget
etc.

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface ClassMatchProcessor {
    public void processMatch(Class<?> klass);
}

public <T> FastClasspathScanner matchClassesWithFieldOfType(Class<T> fieldType,
        ClassMatchProcessor classMatchProcessor)

// Mechanism 2: Call the following after calling .scan():

public List<String> getNamesOfClassesWithFieldOfType(Class<?> fieldType | String fieldTypeName)

8. Performing the actual scan

The .scan() method performs the actual scan. This method may be called multiple times after the initialization steps shown above, although there is usually no point performing additional scans unless classpathContentsModifiedSinceScan() returns true.

public void scan()

As the scan proceeds, for all match processors that deal with classfiles (i.e. for all but FileMatchProcessor), if the same fully-qualified class name is encountered more than once on the classpath, the second and subsequent definitions of the class are ignored, in order to follow Java's class masking behavior.

9. Detecting changes to classpath contents after the scan

When the classpath is scanned using .scan(), the "latest last modified timestamp" found anywhere on the classpath is recorded (i.e. the latest timestamp out of all last modified timestamps of all files found within the whitelisted package prefixes on the classpath).

After a call to .scan(), it is possible to later call .classpathContentsModifiedSinceScan() at any point to check if something within the classpath has changed. This method does not look inside classfiles and does not call any match processors, but merely looks at the last modified timestamps of all files and zip/jarfiles within the whitelisted package prefixes of the classpath, updating the latest last modified timestamp if anything has changed. If the latest last modified timestamp increases, this method will return true.

Since .classpathContentsModifiedSinceScan() only checks file modification timestamps, it works several times faster than the original call to .scan(). It is therefore a very lightweight operation that can be called in a polling loop to detect changes to classpath contents for hot reloading of resources.

The function .classpathContentsLastModifiedTime() can also be called after .scan() to find the maximum timestamp of all files in the classpath, in epoch millis. This should be less than the system time, and if anything on the classpath changes, this value should increase, assuming the timestamps and the system time are trustworthy and accurate.

public boolean classpathContentsModifiedSinceScan()

public long classpathContentsLastModifiedTime()

If you need more careful change detection than is afforded by checking timestamps, you can also cause the contents of each classfile in a whitelisted package to be MD5-hashed, and you can compare the HashMaps returned across different scans.

10. Get a list of all whitelisted (and non-blacklisted) classes, interfaces or annotations on the classpath

The names of all classes, interfaces and/or annotations in whitelisted (and non-blacklisted) packages can be returned using the methods shown below. Note that system classes (e.g. java.lang.String and java.lang.Object) are not enumerated or returned by any of these methods.

// Mechanism 1: Attach a MatchProcessor before calling .scan():

@FunctionalInterface
public interface ClassMatchProcessor {
    public void processMatch(Class<?> klass);
}

// Enumerate all standard classes, interfaces and annotations
public FastClasspathScanner matchAllClasses(
    ClassMatchProcessor classMatchProcessor)

// Enumerate all standard classes (but not interfaces/annotations)
public FastClasspathScanner matchAllStandardClasses(
    ClassMatchProcessor classMatchProcessor)

// Enumerate all interfaces
public FastClasspathScanner matchAllInterfaceClasses(
    ClassMatchProcessor classMatchProcessor)

// Enumerate all annotations
public FastClasspathScanner matchAllAnnotationClasses(
    ClassMatchProcessor classMatchProcessor)

// Mechanism 2: Call one of the following after calling .scan():

// Get names of all standard classes, interfaces and annotations
public List<String> getNamesOfAllClasses()

// Get names of all standard classes (but not interfaces/annotations)
public List<String> getNamesOfAllStandardClasses()

// Get names of all interfaces
public List<String> getNamesOfAllInterfaceClasses()

// Get names of all annotations
public List<String> getNamesOfAllAnnotationClasses()

11. Get all unique directories and files on the classpath

The list of all directories and files on the classpath is returned by .getUniqueClasspathElements(). The resulting list is filtered to include only unique classpath elements (duplicates are eliminated), and to include only directories and files that actually exist. The elements in the list are in classpath order.

This method is useful if you want to see what's actually on the classpath -- note that System.getProperty("java.class.path") does not always return the complete classpath because Classloading is a very complicated process. FastClasspathScanner looks for classpath entries in java.class.path and in various system classloaders, but it can also transitively follow Class-Path references in a jarfile's META-INF/MANIFEST.MF.

Note that FastClasspathScanner does not scan JRE system, bootstrap or extension jarfiles, so the classpath entries for these system jarfiles will not be listed by .getUniqueClasspathElements().

public List<File> getUniqueClasspathElements()

12. Generate a GraphViz dot file from the classgraph

During scanning, the class graph (the connectivity between classes, interfaces and annotations) is determined for all whitelisted (non-blacklisted) packages. The class graph can very simply be turned into a GraphViz .dot file for visualization purposes, as shown above.

Call the following after .scan(), where the sizeX and sizeY params give the layout size in inches:

public String generateClassGraphDotFile(float sizeX, float sizeY)

The returned string can be saved to a .dot file and fed into GraphViz using

dot -Tsvg < graph.dot > graph.svg

or similar, generating a graph with the following conventions:

Note: Graph nodes will only be added for classes, interfaces and annotations that are within whitelisted (non-blacklisted) packages. In particular, the Java standard libraries are excluded from classpath scanning for efficiency, so these classes will never appear in class graph visualizations.

Debugging

If FastClasspathScanner is not finding the classes, interfaces or files you think it should be finding, you can debug the scanning behavior by calling .verbose() before .scan():

public FastClasspathScanner verbose()

More complex usage

Working in platforms with non-standard ClassLoaders (JBoss/WildFly, WebLogic, Maven, Tomcat etc.)

FastClasspathScanner handles a number of non-standard ClassLoaders. There is basic support for JBoss/WildFly and WebLogic, which implement their own ClassLoaders. Maven works when it sets java.class.path, but YMMV, since it has its own unique ClassLoader system. Tomcat has a complex classloading system, and is less likely to work, but you might get lucky. You can add custom ClassLoader handlers to your project without modifying FastClasspathScanner if necessary, although patches that add or improve support for common non-standard ClassLoaders would be appreciated.

Note that you can always override the system classpath with your own path, using the following call after calling the constructor, and before calling .scan():

public FastClasspathScanner overrideClasspath(String classpath)

Getting generic class references for parameterized classes

A problem arises when using class-based matchers with parameterized classes, e.g. Widget<K>. Because of type erasure, The expression Widget<K>.class is not defined, and therefore it is impossible to cast Class<Widget> to Class<Widget<K>>. More specifically:

Widget.class has the type Class<Widget>, not Class<Widget<?>>
new Widget<Integer>().getClass() has the type Class<? extends Widget>, not Class<? extends Widget<?>>.

The code below compiles and runs fine, but SubclassMatchProcessor must be parameterized with the bare type Widget in order to match the reference Widget.class. This gives rise to three type safety warnings: Test.Widget is a raw type. References to generic type Test.Widget<K> should be parameterized on new SubclassMatchProcessor<Widget>() and Class<? extends Widget> widgetClass; and Type safety: Unchecked cast from Class<capture#1-of ? extends Test.Widget> to Class<Test.Widget<?>> on the type cast (Class<? extends Widget<?>>).

public class Test {
    public static class Widget<K> {
        K id;
    }

    public static class WidgetSubclass<K> extends Widget<K> {
    }  

    public static void registerSubclass(Class<? extends Widget<?>> widgetClass) {
        System.out.println("Found widget subclass " + widgetClass.getName());
    }
    
    public static void main(String[] args) {
        new FastClasspathScanner("com.xyz.widget")
            // Have to use Widget.class and not Widget<?>.class as type parameter,
            // which constrains all the other types to bare class references
            .matchSubclassesOf(Widget.class, new SubclassMatchProcessor<Widget>() {
                @Override
                public void processMatch(Class<? extends Widget> widgetClass) {
                    registerSubclass((Class<? extends Widget<?>>) widgetClass);
                }
            })
            .scan();
    }
}

Solution: You can't cast from Class<Widget> to Class<Widget<?>>, but you can cast from Class<Widget> to Class<? extends Widget<?>> with only an unchecked conversion warning, which can be suppressed.

[The gory details: The type Class<? extends Widget<?>> is unifiable with the type Class<Widget<?>>, so for the method matchSubclassesOf(Class<T> superclass, SubclassMatchProcessor<T> subclassMatchProcessor), you can use <? extends Widget<?>> for the type parameter <T> of superclass, and type <Widget<?>> for the type parameter <T> of subclassMatchProcessor.]

Note that with this cast, SubclassMatchProcessor<Widget<?>> can be properly parameterized to match the type of widgetClassRef, and no cast is needed in the function call registerSubclass(widgetClass).

(Also note that it is valid to replace all occurrences of the generic type parameter <?> in this example with a concrete type parameter, e.g. <Integer>.)

public static void main(String[] args) {
    // Declare the type as a variable so you can suppress the warning
    @SuppressWarnings("unchecked")
    Class<? extends Widget<?>> widgetClassRef = 
        (Class<? extends Widget<?>>) Widget.class;
    new FastClasspathScanner("com.xyz.widget").matchSubclassesOf(
                widgetClassRef, new SubclassMatchProcessor<Widget<?>>() {
            @Override
            public void processMatch(Class<? extends Widget<?>> widgetClass) {
                registerSubclass(widgetClass);
            }
        })
        .scan();
}

Alternative solution 1: Create an object of the desired type, call getClass(), and cast the result to the generic parameterized class type.

public static void main(String[] args) {
    @SuppressWarnings("unchecked")
    Class<Widget<?>> widgetClass = 
        (Class<Widget<?>>) new Widget<Object>().getClass();
    new FastClasspathScanner("com.xyz.widget") //
        .matchSubclassesOf(widgetClass, new SubclassMatchProcessor<Widget<?>>() {
            @Override
            public void processMatch(Class<? extends Widget<?>> widgetClass) {
                registerSubclass(widgetClass);
            }
        })
        .scan();
}

Alternative solution 2: Get a class reference for a subclass of the desired class, then get the generic type of its superclass:

public static void main(String[] args) {
    @SuppressWarnings("unchecked")
    Class<Widget<?>> widgetClass =
            (Class<Widget<?>>) ((ParameterizedType) WidgetSubclass.class
                .getGenericSuperclass()).getRawType();
    new FastClasspathScanner("com.xyz.widget") //
        .matchSubclassesOf(widgetClass, new SubclassMatchProcessor<Widget<?>>() {
            @Override
            public void processMatch(Class<? extends Widget<?>> widgetClass) {
                registerSubclass(widgetClass);
            }
        })
        .scan();
}

License

The MIT License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Classfile format documentation

See Oracle's documentation on the classfile format.

Inspiration

FastClasspathScanner was inspired by Ronald Muller's annotation-detector.

Alternatives

Reflections could be a good alternative if Fast Classpath Scanner doesn't meet your needs.

Author

Fast Classpath Scanner was written by Luke Hutchison -- https://github.com/lukehutch

Please if this library makes your life easier.

regis-leray/fast-classpath-scanner