目录

• 本文debug slf4j 的源码执行流程
• 依次执行
  • 不添加 日志实现框架（本篇主讲）
  • 添加日志实现框架
• 通晓Slf4j 内部实现根据 classpath 实现 facade 的原理

涉及知识点

• 类加载
• 设计模式
  • factory 工厂模式
  • spi service provider interface 模式
  • facade 外观模式
  • singleton 单例模式

前沿

• commons-logging和slf4j都是日志的接口，供用户使用，而没有提供实现
• 当年Apache说服log4j以及其他的日志来按照commons-logging的标准编写，但是由于commons-logging的类加载有点问题，实现起来也不友好，因此log4j的作者就创作了slf4j，也因此而与commons-logging两分天下

项目背景

• IJ Maven 项目
• 准备 pom.xml

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.25</version>
</dependency>

• 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：进入 LoggerFactory.java 之 getLogger(Class clazz)
  • 首先注意 LoggerFactory 有一个 静态变量 INITIALIZATION_STATE，标记ILoggerFactory 实例初始化的结果
  • 该变量的可能值已经用 static final 变量分别定义

//INITIALIZATION_STATE 的可能结果：

static final int UNINITIALIZED = 0;
static final int ONGOING_INITIALIZATION = 1;
static final int FAILED_INITIALIZATION = 2;
static final int SUCCESSFUL_INITIALIZATION = 3;
static final int NOP_FALLBACK_INITIALIZATION = 4;

1）类加载时初始化该变量

//类加载 初始化为 UNINITIALIZED 0
static volatile int INITIALIZATION_STATE = UNINITIALIZED;

2）现在进入 getLogger(Class clazz)

/**
 * Return a logger named corresponding to the class passed as parameter,
 * using the statically bound ILoggerFactory instance. 
 * 根据静态绑定的 ILoggerFactory 实例 获取 logger
 **/
public static Logger getLogger(Class<?> clazz) {
    
    //核心代码 class=log.learn.LogDemo
    Logger logger = getLogger(clazz.getName());
    
    if (DETECT_LOGGER_NAME_MISMATCH) {
        Class<?> autoComputedCallingClass = Util.getCallingClass();
        if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
            Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                            autoComputedCallingClass.getName()));
            Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
        }
    }
    return logger;
}

• step3: 进入同类 getLogger(String name)
  • 该方法是一个收口，以String 为参 获取和以 Class 类型为参都转至此

/**
 * Return a logger named according to the name parameter using the
 * statically bound ILoggerFactory instance.
 * 说明最终获取 logger 的实现是由 静态绑定的 ILoggerFactory 实例决定的
 **/
public static Logger getLogger(String name) {
    //进入 getILoggerFactory 方法
    ILoggerFactory iLoggerFactory = getILoggerFactory();
    return iLoggerFactory.getLogger(name);
}

• step4：进入同类 getILoggerFactory()

/**
 * Return the ILoggerFactory instance in use.
 * 
 * ILoggerFactory instance is bound with this class at compile time.
 * 在编译时刻绑定 ILoggerFactory 的具体实例
 **/
public static ILoggerFactory getILoggerFactory() {

    //类加载时， INITIALIZATION_STATE == UNINITIALIZED，标志为未初始化
    //且对 初始化状态的更新使用了 双重校验锁（DCL，即 double-checked locking），参考单例模式初始化
    if (INITIALIZATION_STATE == UNINITIALIZED) {
        synchronized (LoggerFactory.class) {
            if (INITIALIZATION_STATE == UNINITIALIZED) {
                //将初始化状态标记为 ONGING
                INITIALIZATION_STATE = ONGOING_INITIALIZATION;ONGING
                //执行 ILoggerFactory 实例初始化，实际是开始在 classpath 寻找看看是否存在 。。。
                //我们先跳到初始化的位置，这也是Slf4j 实现 facade 的核心所在
                //了解了核心，再转回来看下面的
                performInitialization();
            }
        }
    }
    switch (INITIALIZATION_STATE) {
    case SUCCESSFUL_INITIALIZATION:
        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");
}

• step5: 进入performInitialization 方法

//可以看到该方法用 private 修饰，即在类内部使用
private final static void performInitialization() {
    //核心代码 bind, 开始绑定 ILoggerFactory 具体实现
    bind();
    if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
        versionSanityCheck();
    }
}

• step6: 进入 bind 方法

private final static void bind() {
    try {
        //存放
        Set<URL> staticLoggerBinderPathSet = null;
        // skip check under android, see also
        // http://jira.qos.ch/browse/SLF4J-328
        if (!isAndroid()) {
            //绑定的过程，该方法去寻找 StaticLoggerBinder.class 文件
            //Step7: 折返回来向下执行
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
        // the next line does the binding
        //寻找 StaticLoggerBinder 类若找不到则抛异常
        //pom 没有添加 日志实现依赖，抛异常
        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)) {
            //初始化状态为 
            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);
    }
}

无slf4j-simple.jar 时打出的日志

Connected to the target VM, address: '127.0.0.1:64503', transport: 'socket'
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

• step7: 进入 findPossibleStaticLoggerBinderPathSet 方法

// We need to use the name of the StaticLoggerBinder class, but we can't
// reference the class itself.
// 根据classpath 下是否存在 StaticLoggerBinder 来作为判断是否存在 具体日志实现框架的标准
// 此处也暗示出 所有日志实现框架的包路径及 所必需包含的 StaticLoggerBinder 类路径
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;
        
        
        //而用户在运行期，是获取不到引导类加载器bootstrapclassloader的，因此当一个类获取它的类加载器，得到的对象时null，就说明它是由引导类加载器加载的。
        //引导类加载器是负责加载系统目录下的文件，因此源码中使用getSystemresource来获取资源文件。
        if (loggerFactoryClassLoader == null) {
            paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
        } else {
            //判断能否找到 StaticLoggerBinder 的 class 文件
            //pom 未添加 日志实现 依赖包的话 是找不到该 class 文件的
            //因为可能存在若干个该 class 文件，故此处用 Enumeration 来迭代存储URL，Enumeration 现在被 Iteration 替代
            //两者功能一致
            paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
        }
        while (paths.hasMoreElements()) {
            URL path = paths.nextElement();
            //将所有class url 存到有序set 中
            staticLoggerBinderPathSet.add(path);
        }
    } catch (IOException ioe) {
        Util.report("Error getting resources from path", ioe);
    }
    //返回保存了class URL 的有序集合，转到 step6,继续向下执行
    return staticLoggerBinderPathSet;
}

• 重新研究 step6 后半部分

private final static void bind() {
    try {
        //存放
        Set<URL> staticLoggerBinderPathSet = null;
        // skip check under android, see also
        // http://jira.qos.ch/browse/SLF4J-328
        if (!isAndroid()) {
            //绑定的过程，该方法去寻找 StaticLoggerBinder.class 文件
            //Step7: 折返回来向下执行
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
        // the next line does the binding
        //寻找 StaticLoggerBinder 类若找不到则抛异常
        //pom 没有添加 日志实现依赖，抛异常
        //此处在pom未添加 日志实现类时，classpath 是不存在 StaticLoggerBinder class 的，故抛出NoClassDefFoundError 异常
        //进入catch 代码块
        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)) {
            //初始化状态为 NOP_FALLBACK_INITIALIZATION，
            //bind 方法结束,step5 中 performInitialization 方法结束
            //INITIALIZATION_STATE 决定 后续使用哪个日志实现框架
            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);
    }
}

• step8: 进入 getILoggerFactory() 的 switch

//即最终选择的日志实现框架是通过 INITIALIZATION_STATE 来区分的
switch (INITIALIZATION_STATE) {
//如果是 SUCCESSFUL_INITIALIZATION，则说明成功在 classpath 找到了 实现框架
case SUCCESSFUL_INITIALIZATION:
    return StaticLoggerBinder.getSingleton().getLoggerFactory();

//说明未找到
case NOP_FALLBACK_INITIALIZATION:
    return NOP_FALLBACK_FACTORY;

//寻找过程出现异常
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;
}

• step9：重新进入 getLogger(String name)

public static Logger getLogger(String name) {
    //classpath 不存在日志实现框架的，此刻得到的是 NOPLoggerFactory 的实例
    //
    ILoggerFactory iLoggerFactory = getILoggerFactory();
    //进入NOPLoggerFactory 的 getLogger 
    return iLoggerFactory.getLogger(name);
}

• step10：进入NOPLoggerFactory 的 getLogger 方法

public class NOPLoggerFactory implements ILoggerFactory {

    public NOPLoggerFactory() {
        // nothing to do
    }
    
    //获取具体日志对象
    public Logger getLogger(String name) {
        return NOPLogger.NOP_LOGGER;
    }

}

• Step11：进入 NOPLogger.NOP_LOGGER

public class NOPLogger extends MarkerIgnoringBase {

    private static final long serialVersionUID = -517220405410904473L;

    /**
     * The unique instance of NOPLogger.
     * 获得 NOPLogger 实例
     */
    public static final NOPLogger NOP_LOGGER = new NOPLogger();
    
    ......
    
    //实现的Logger 的所有方法，方法体全为空
    /** A NOP implementation.  */
    public final void debug(String format, Object arg1, Object arg2) {
        // NOP
    }
}

• Step12：重新进入main 方法

public class LogDemo {
    public static void main(String[] args) {
        Logger logger = LoggerFactory.getLogger(LogDemo.class);
        //进入NOPLogger 的 info 方法，方法体为空
        logger.info("log");
    }
}