This requires Java 22 + --enable-preview to compile and run since
- this relies on StringTemplate preview language feature
- this relies on ClassFileAPI preview API
$ javac --enable-preview -source 22
# Run ClassFileGenerator first to create Main.class with indy for StringTemplate Processor invocation
$ java --enable-preview ClassFileGenerator
# ClassFileGenerator generates Main.class in /tmp/Main.class
# Copy it to root dir of repo
$ cp /tmp/Main.class .
# Then run Main
$ java --enable-preview Main
Create new processor, some processor may parse fragments, so slow
Process string template
Hello duke
Process string template
Hello duke
Process string template
Hello duke
# Main calls doStringTemplate 3 times, and doStringTemplate runs StringTemplate expression
# However, you see "Create new processor, some processor may parse fragments, so slow" parsing of fragments are done only once in first call thanks to invokedynamic and caching
# We can change this behavior
# Modify StringTemplateSTRDebug.java like
$ git diff
diff --git a/StringTemplateSTRDebug.java b/StringTemplateSTRDebug.java
index 189627a..b7a2e23 100644
--- a/StringTemplateSTRDebug.java
+++ b/StringTemplateSTRDebug.java
@@ -15,6 +15,6 @@ public class StringTemplateSTRDebug implements StringTemplateProcessorFactory {
@Override
public boolean cacheProcessor() {
- return true;
+ return false;
}
}
$ javac --enable-preview --source 22 StringTemplateSTRDebug.java
$ java --enable-preview Main
Create new processor, some processor may parse fragments, so slow
Process string template
Hello duke
Create new processor, some processor may parse fragments, so slow
Process string template
Hello duke
Create new processor, some processor may parse fragments, so slow
Process string template
Hello duke
# Now you see fragments are parsed 3 times (inefficient)
Unlike proposed StringTemplate.Processor in JEP 459, StringTemplateProcessorFactory responsibility is creating StringTemplateProcessor from fragments of StringTemplate. StringTemplateProcessor does creating result of StringTemplate expression Object/T from Object[] (StringTemplate#values). So I split the responsibility and thanks to it, we can separate the parsing StringTemplate#fragments and appling StringTemplate#values.
Parsing StringTemplate#fragments could be heavy than appling StringTemplate#value, so parsing fragments only once, and cache the parsed result is important to improve the performance.
Now we can cache StringTemplateProcessor for StringTemplate expression by paring StringTemplateRuntime to the StringTemplate expression and thanks to invokedynamic. Overriding StringTemplateProcessorFactory#cacheProcessor let implementations to allow caching StringTemplateProcessor or not.
StringTemplateSTRDebug is a implementation of StringTemplate.STR with debug print in case of StringTemplate processing is implemented with StringTemplateProcessorFactory and StringTemplateProcessor.
- when parsing fragments occurred (StringTemplateProcessorFactory#createProcessor is called) (actually STR doesn't heavy work for fragments, but assume this is a StringTemplate for JSON or DB query builder)
- when processing values for the fragments occurred (StringTemplateProcessor is called)
StringTemplateRuntime calls StringTemplateProcessorFactory and StringTemplateProcessor for actual StringTemplate fragments and values. StringTemplateRuntime cache StringTemplateProcessor if StringTemplateProcessorFactory allows it.
StringTemplateRuntime is created for each of StringTemplate expression. So
- StringTemplateRuntime instance for
STR."Hello, \{name}"caches StringTemplateProcessor created by STR.createProcessor(new String[] ("Hello, ", "")) - StringTemplateRuntime instance for
STR."Java \{version}"caches StringTemplateProcessor created by STR.createProcessor(new String[] ("Java ", ""))
StringTemplateRuntime is paired to the expression by invokedynamic and ConstantCallSite
StringTemplateBSM is a BSM for invokedynamic call for StringTemplate expression.
- Create new StringTemplateBSM instance.
- Create MethodHandle for StringTemplateBSM#process(StringTemplateProcessorFactory, String[], Object[]): Object
- Bind the StringTemplateBSM instance to the MethodHandle receiver
ClassFileGenerator generate Main.class using ClassFileAPI with invokedynamic for StringTemplate. Assuming Main.java is
public class Main {
public static void doStringTemplate(String[] args) throws Throwable {
String name = "duke";
System.out.println(StringTemplateSTRDebug.STR."Hello \{name}");
}
public static void main(String[] args) throws Throwable {
doStringTemplate();
doStringTemplate();
doStringTemplate();
}
}
(It's not fully correspond byte codes to above codes)
Actual generated (human readable by javap) byte codes are
Last modified 2024/03/28; size 829 bytes
SHA-256 checksum 1c6fafab0b8e764729f468d554b8b001236b21a922b3ee8668fcbd84130e4daf
public class Main
minor version: 0
major version: 66
flags: (0x0001) ACC_PUBLIC
this_class: #2 // Main
super_class: #18 // java/lang/Object
interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
#1 = Utf8 Main
#2 = Class #1 // Main
#3 = Utf8 doStringTemplate
#4 = Utf8 ()V
#5 = Utf8 StringTemplateSTRDebug
#6 = Class #5 // StringTemplateSTRDebug
#7 = Utf8 STR
#8 = Utf8 LStringTemplateProcessorFactory;
#9 = NameAndType #7:#8 // STR:LStringTemplateProcessorFactory;
#10 = Fieldref #6.#9 // StringTemplateSTRDebug.STR:LStringTemplateProcessorFactory;
#11 = Utf8 java/lang/String
#12 = Class #11 // java/lang/String
#13 = Utf8 Hello
#14 = String #13 // Hello
#15 = Utf8
#16 = String #15 //
#17 = Utf8 java/lang/Object
#18 = Class #17 // java/lang/Object
#19 = Utf8 duke
#20 = String #19 // duke
#21 = Utf8 StringTemplateBSM
#22 = Class #21 // StringTemplateBSM
#23 = Utf8 createStringTemplateRuntimeCallSite
#24 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#25 = NameAndType #23:#24 // createStringTemplateRuntimeCallSite:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#26 = Methodref #22.#25 // StringTemplateBSM.createStringTemplateRuntimeCallSite:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#27 = MethodHandle 6:#26 // REF_invokeStatic StringTemplateBSM.createStringTemplateRuntimeCallSite:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#28 = Utf8 process
#29 = Utf8 (LStringTemplateProcessorFactory;[Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/Object;
#30 = NameAndType #28:#29 // process:(LStringTemplateProcessorFactory;[Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/Object;
#31 = InvokeDynamic #0:#30 // #0:process:(LStringTemplateProcessorFactory;[Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/Object;
#32 = Utf8 java/lang/System
#33 = Class #32 // java/lang/System
#34 = Utf8 out
#35 = Utf8 Ljava/io/PrintStream;
#36 = NameAndType #34:#35 // out:Ljava/io/PrintStream;
#37 = Fieldref #33.#36 // java/lang/System.out:Ljava/io/PrintStream;
#38 = Utf8 java/io/PrintStream
#39 = Class #38 // java/io/PrintStream
#40 = Utf8 println
#41 = Utf8 (Ljava/lang/Object;)V
#42 = NameAndType #40:#41 // println:(Ljava/lang/Object;)V
#43 = Methodref #39.#42 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#44 = Utf8 main
#45 = Utf8 ([Ljava/lang/String;)V
#46 = NameAndType #3:#4 // doStringTemplate:()V
#47 = Methodref #2.#46 // Main.doStringTemplate:()V
#48 = Utf8 Code
#49 = Utf8 BootstrapMethods
{
public static void doStringTemplate();
descriptor: ()V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=6, locals=2, args_size=0
0: getstatic #10 // Field StringTemplateSTRDebug.STR:LStringTemplateProcessorFactory;
3: iconst_2
4: anewarray #12 // class java/lang/String
7: dup
8: iconst_0
9: ldc #14 // String Hello
11: aastore
12: dup
13: iconst_1
14: ldc #16 // String
16: aastore
17: iconst_1
18: anewarray #18 // class java/lang/Object
21: dup
22: iconst_0
23: ldc #20 // String duke
25: aastore
26: invokedynamic #31, 0 // InvokeDynamic #0:process:(LStringTemplateProcessorFactory;[Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/Object;
31: astore_1
32: getstatic #37 // Field java/lang/System.out:Ljava/io/PrintStream;
35: aload_1
36: invokevirtual #43 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
39: return
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=0, locals=1, args_size=1
0: invokestatic #47 // Method doStringTemplate:()V
3: invokestatic #47 // Method doStringTemplate:()V
6: invokestatic #47 // Method doStringTemplate:()V
9: return
}
BootstrapMethods:
0: #27 REF_invokeStatic StringTemplateBSM.createStringTemplateRuntimeCallSite:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments: