主题

• 本篇是Slf4j 源码解析的第二篇，上篇介绍了无日志实现框架的执行流程
• 本篇解析添加 slf4j-simple 日志实现框架的脉络
• 已经知道：org/sfl4j/impl/StaticLoggerBinder.class 是连接日志实现框架和Slf4j facade 的桥梁

先浏览下slf4j-simple.jar的目录结构

• 关键是探求如何与Slf4j-api.jar facade 关联的，实现框架项目的复杂简单 not care

//如下是 slf4j-simple.jar 项目的结构示意图
//最主要的就是 org.slf4j.impl 包路径

slf4j-simple 
    src
        main
            java //sources
                org.slf4j.impl //package
                    SimpleLogger.java
                    SimpleLoggerFactory.java
                    StaticLoggerBinder.java
                    SimpleLoggerConfiguration.java
                    ......
            resources
                
    ......
    ......
    pom.xml

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>
    
    <parent>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-parent</artifactId>
    <version>1.7.25</version>
    </parent>
    
    <artifactId>slf4j-simple</artifactId>
    <packaging>jar</packaging>
    <name>SLF4J Simple Binding</name>
    <description>SLF4J Simple binding</description>
    <url>http://www.slf4j.org</url>
    
    
    <dependencies>
        <!--添加了slf4j-api依赖，因为要继承其各种接口，如 Logger, ILoggerFactory 等-->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
        </dependency>
        
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <type>test-jar</type>
            <version>${project.version}</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

</project>

详细流程

• step1：入口

public class LogDemo {
    public static void main(String[] args) {
        Logger logger = LoggerFactory.getLogger(LogDemo.class); //断点处
        logger.info("log");
        System.out.println("log test");
    }
}

• Step2：中间跳过与上一篇文章相同之处，进入寻找 StaticLoggerBinder.class方法

// We need to use the name of the StaticLoggerBinder class, but we can't
// reference
// the class itself.
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";

static Set<URL> findPossibleStaticLoggerBinderPathSet() {
    // use Set instead of list in order to deal with bug #138
    // LinkedHashSet appropriate here because it preserves insertion order
    // during iteration
    Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
    try {
        ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
        Enumeration<URL> paths;
        if (loggerFactoryClassLoader == null) {
            paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
        } else {
            //此处是关键能找到 StaticLoggerFactory.class 文件
            //file:/D:/local/mvnrepo/org/slf4j/slf4j-simple/1.7.25/slf4j-simple-1.7.25.jar!/org/slf4j/impl/StaticLoggerBinder.class
            //paths.size() == 1
            paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
        }
        while (paths.hasMoreElements()) {
            URL path = paths.nextElement();
            staticLoggerBinderPathSet.add(path);
        }
    } catch (IOException ioe) {
        Util.report("Error getting resources from path", ioe);
    }
    //携带者StaticLoogerBinder.class URL 返回
    return staticLoggerBinderPathSet;
}

• Step3：进入bind()

private final static void bind() {
    try {
        Set<URL> staticLoggerBinderPathSet = null;
        // skip check under android, see also
        // http://jira.qos.ch/browse/SLF4J-328
        if (!isAndroid()) {
            //Step2 中已经找到，向下执行
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
        // the next line does the binding
        //因为Step2 中明确已经找到 StaticLoggerBinder class,故此时不会抛异常，进入getSingleton()
        StaticLoggerBinder.getSingleton();
        INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstituteLoggers();
        replayEvents();
        // release all resources in SUBST_FACTORY
        SUBST_FACTORY.clear();
    } catch (NoClassDefFoundError ncde) {
        String msg = ncde.getMessage();
        if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
            INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
            Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
            Util.report("Defaulting to no-operation (NOP) logger implementation");
            Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
        } else {
            failedBinding(ncde);
            throw ncde;
        }
    } catch (java.lang.NoSuchMethodError nsme) {
        String msg = nsme.getMessage();
        if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
            INITIALIZATION_STATE = FAILED_INITIALIZATION;
            Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
            Util.report("Your binding is version 1.5.5 or earlier.");
            Util.report("Upgrade your binding to version 1.6.x.");
        }
        throw nsme;
    } catch (Exception e) {
        failedBinding(e);
        throw new IllegalStateException("Unexpected initialization failure", e);
    }
}

• Step4：进入StaticLoggerBinder.getSingleton()
  • StaticLoggerBinder 定义于slf4j-simple.jar 中

package org.slf4j.impl; //slf4j-api.jar 中定义的static StaticLoggerBinder.class 包路径

import org.slf4j.ILoggerFactory; //slf4j-api的包
import org.slf4j.spi.LoggerFactoryBinder;  //slf4j-api的包

public class StaticLoggerBinder implements LoggerFactoryBinder {

    //这里用了饿汉单例模式生成 StaticLoggerBinder 类的实例
    private static final StaticLoggerBinder SINGLETON = new StaticLoggerBinder();
    public static String REQUESTED_API_VERSION = "1.6.99";
    private static final String loggerFactoryClassStr = SimpleLoggerFactory.class.getName();
    
    //每个StaticLoggerBinder 绑定了一个 ILoggerFactory 用以返回具体的 LoggerFactory 此处绑定的是 SimpleLoggerFactory
    private final ILoggerFactory loggerFactory = new SimpleLoggerFactory();

    //返回 StaticLoggerBinder 单例
    public static final StaticLoggerBinder getSingleton() {
        return SINGLETON;
    }

    private StaticLoggerBinder() {
    }

    //返回日志工厂
    public ILoggerFactory getLoggerFactory() {
        return this.loggerFactory;
    }

    public String getLoggerFactoryClassStr() {
        return loggerFactoryClassStr;
    }
}

• Step5：返回 bind 方法

private final static void bind() {
    try {
        Set<URL> staticLoggerBinderPathSet = null;
        // skip check under android, see also
        // http://jira.qos.ch/browse/SLF4J-328
        if (!isAndroid()) {
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
        // the next line does the binding
        //成功执行
        StaticLoggerBinder.getSingleton();
        // INITIALIZATION_STATE 赋值成功
        // bind 结束
        INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstituteLoggers();
        replayEvents();
        // release all resources in SUBST_FACTORY
        SUBST_FACTORY.clear();
    } catch (NoClassDefFoundError ncde) {
        String msg = ncde.getMessage();
        if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
            INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
            Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
            Util.report("Defaulting to no-operation (NOP) logger implementation");
            Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
        } else {
            failedBinding(ncde);
            throw ncde;
        }
    } catch (java.lang.NoSuchMethodError nsme) {
        String msg = nsme.getMessage();
        if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
            INITIALIZATION_STATE = FAILED_INITIALIZATION;
            Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
            Util.report("Your binding is version 1.5.5 or earlier.");
            Util.report("Upgrade your binding to version 1.6.x.");
        }
        throw nsme;
    } catch (Exception e) {
        failedBinding(e);
        throw new IllegalStateException("Unexpected initialization failure", e);
    }
}

• Step：返回 performInitialization

private final static void performInitialization() {
    bind();
    if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
        versionSanityCheck();
    }
}

• Step：返回 getILoggerFactory()

public static ILoggerFactory getILoggerFactory() {
    if (INITIALIZATION_STATE == UNINITIALIZED) {
        synchronized (LoggerFactory.class) {
            if (INITIALIZATION_STATE == UNINITIALIZED) {
                INITIALIZATION_STATE = ONGOING_INITIALIZATION;
                //执行结束
                performInitialization();
            }
        }
    }
    //进入switch 
    switch (INITIALIZATION_STATE) {
    //此时INITIALIZATION_STATE 成功初始化
    case SUCCESSFUL_INITIALIZATION:
        //拿到LoggerFactory,然后返回
        return StaticLoggerBinder.getSingleton().getLoggerFactory();
    case NOP_FALLBACK_INITIALIZATION:
        return NOP_FALLBACK_FACTORY;
    case FAILED_INITIALIZATION:
        throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
    case ONGOING_INITIALIZATION:
        // support re-entrant behavior.
        // See also http://jira.qos.ch/browse/SLF4J-97
        return SUBST_FACTORY;
    }
    throw new IllegalStateException("Unreachable code");
}

• Step：

public static Logger getLogger(String name) {
    //取得LoggerFactory
    ILoggerFactory iLoggerFactory = getILoggerFactory();
    //通过LoggerFactory 得到具体日志框架
    return iLoggerFactory.getLogger(name);
}

• Step：返回main

    public static void main(String[] args) {
        Logger logger = LoggerFactory.getLogger(LogDemo.class);
        logger.info("log");

        URL ls = Logger.class.getClassLoader().getResource("org/slf4j/Logger.class");
        System.out.println("log test");
    }