Learn Java Log System

Project Structure

├─java-log-impl           # Java some log implementations（jul、log4j、log4j2、logback）
├─java-log-api-jcl        # commons logging
├─java-log-api-slf4j      # slf4j
└─log4j2-vulnerabilities  # log4j2 CVE recurrence

Log4j2 CVE-2021-44228 recurrence step

cd log4j2-vulnerabilities
run com.itplh.log4j2.RmiServer
run com.itplh.log4j2.TestLog4j2UseJndi

Java Log System Implementation plan

Interface
- JCL（Apache Commons Logging）
- Slf4j
Implementation
- JUL（java.util.logging）
- log4j
- log4j2
- logback

JCL（Apache Commons Logging, alias Jakarta Commons Logging）

cd java-log-api-jcl
Run com.itplh.jcl.TestCommonsLogging, then observe console output
Adjust org.apache.commons.logging.Log's value in commons-logging.properties, then rerun program and observe console output

Principle

Load implement class who org.apache.commons.logging.Log, the bind log framework used it, invoke stack as follows：

Log log = LogFactory.getLog("jcl");
    |- getLog:669, LogFactory (org.apache.commons.logging)
        |- getInstance:292, LogFactoryImpl (org.apache.commons.logging.impl)
            |- newInstance:541, LogFactoryImpl (org.apache.commons.logging.impl)
                |- discoverLogImplementation:790, LogFactoryImpl (org.apache.commons.logging.impl)
                    |- createLogFromClass:960, LogFactoryImpl (org.apache.commons.logging.impl)

The whole process that commons-logging get Logger, details and summary as follows：

Get current thread of classloader, then classloader get LogFactroy from cache, that cache is WeakHashTable; if the cache exists it, return it, otherwise into next step；
 
Load commons-logging.properties that under classpath, judge use_tccl attribute, if it exists, then judge its value, if value is false, set baseClassLoader as current class classLoader；
 
Then get LogFactory, and has four ways: 
    (1)in system property find “org.apache.commons.logging.LogFactory” of value, then generate LogFactory;
    (2)in resource “META-INF/services/org.apache.commons.logging.LogFactory” of value, then generate LogFactory;
    (3)get “org.apache.commons.logging.LogFactory” of value from commons-logging.properties, then generate LogFactory;
    (4)if none of the above successful, create default log factory：org.apache.commons.logging.impl.LogFactoryImpl

After get log factory is successfly, according to class name  get Log instance;

The main logic is in discoverLogImplementation method:
    (1)Check is exists “org.apache.commons.logging.Log” key in commons-logging.properties, create Log's implement class if this key exists, otherwise execute next logic;
    (2)Traverse classesToDiscover array, this array saved Log's implement class full name and it contains four elements：org.apache.commons.logging.impl.Log4JLogger、org.apache.commons.logging.impl.Jdk14Logger、org.apache.commons.logging.impl.Jdk13LumberjackLogger、org.apache.commons.logging.impl.SimpleLog；
    (3)Accroding to these full class name, load class file in order, then instantiates, final return Log's implementation.(Log's default implement class is Jdk14Logger.)

Slf4j Adaptor Solution

cd java-log-api-slf4j
Run com.itplh.slf4j.TestSlf4j, then observe console output
Change slf4j's adaptor dependence relation where pom.xml, then rerun program and observe console out put

Principle

Load org.slf4j.impl.StaticLoggerBinder from slf4j adaptor jar package, then bind log framework used it, invoke stack as follows：

Logger logger = LoggerFactory.getLogger("slf4j");
    |- getLogger:362, LoggerFactory (org.slf4j)
        |- getILoggerFactory:417, LoggerFactory (org.slf4j)
            |- performInitialization:124, LoggerFactory (org.slf4j)
                |- bind:146, LoggerFactory (org.slf4j)
                    |- findPossibleStaticLoggerBinderPathSet:301, LoggerFactory (org.slf4j)