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+# mybatis 日志源码
+- Author: [HuiFer](https://github.com/huifer)
+- Description: 该文介绍 mybatis 日志相关源码
+## 核心类
+- `org.apache.ibatis.logging.Log`
+- `org.apache.ibatis.logging.LogFactory`
+- 多个日志实现
+    - `org.apache.ibatis.logging.log4j2.Log4j2Impl`
+    - `org.apache.ibatis.logging.slf4j.Slf4jLocationAwareLoggerImpl`
+    - ...
+    
+## 源码流程
+- mybatis 提供了一个日志接口,内容如下.
+```java
+/**
+ * mybatis 的日志接口,提供日志级别
+ * <ol>
+ *     <li>error</li>
+ *     <li>debug</li>
+ *     <li>trace</li>
+ *     <li>warn</li>
+ * </ol>
+ * <p>通过自己定义的接口来实现各大日志框架的内容达到高可用</p>
+ * @author Clinton Begin
+ */
+public interface Log {
+
+    boolean isDebugEnabled();
+
+    boolean isTraceEnabled();
+
+    void error(String s, Throwable e);
+
+    void error(String s);
+
+    void debug(String s);
+
+    void trace(String s);
+
+    void warn(String s);
+}
+```
+- 有了日志接口必然有实现类, mybatis 有`log4j2` 、 `slf4j` 等日志的相关实现 , 下面是`Slf4jImpl`的代码,其他代码也是一样的模式进行初始化就不再重复贴代码了.
+```java
+public class Slf4jImpl implements Log {
+
+    private Log log;
+
+    /**
+     * 创建日志实例
+     * @param clazz
+     */
+    public Slf4jImpl(String clazz) {
+        Logger logger = LoggerFactory.getLogger(clazz);
+
+        if (logger instanceof LocationAwareLogger) {
+            try {
+                // check for slf4j >= 1.6 method signature
+                logger.getClass().getMethod("log", Marker.class, String.class, int.class, String.class, Object[].class, Throwable.class);
+                log = new Slf4jLocationAwareLoggerImpl((LocationAwareLogger) logger);
+                return;
+            } catch (SecurityException | NoSuchMethodException e) {
+                // fail-back to Slf4jLoggerImpl
+            }
+        }
+
+        // Logger is not LocationAwareLogger or slf4j version < 1.6
+        log = new Slf4jLoggerImpl(logger);
+    }
+
+    @Override
+    public boolean isDebugEnabled() {
+        return log.isDebugEnabled();
+    }
+
+    @Override
+    public boolean isTraceEnabled() {
+        return log.isTraceEnabled();
+    }
+
+    @Override
+    public void error(String s, Throwable e) {
+        log.error(s, e);
+    }
+
+    @Override
+    public void error(String s) {
+        log.error(s);
+    }
+
+    @Override
+    public void debug(String s) {
+        log.debug(s);
+    }
+
+    @Override
+    public void trace(String s) {
+        log.trace(s);
+    }
+
+    @Override
+    public void warn(String s) {
+        log.warn(s);
+    }
+
+}
+
+```
+- 通过上述方法来达到统一接口多个实现,这个在开发中也经常使用.多日志的实现方法有了还缺一个创建方法,创建方法由`org.apache.ibatis.logging.LogFactory`提供
+```java
+
+/**
+ * <p>日志工厂，实现内容:</p>
+ * <ol>
+ *     <li>org.slf4j.Logger 日志框架 slf4j</li>
+ *     <li>org.apache.commons.logging.Log 日志框架 apache</li>
+ *     <li>org.apache.logging.log4j.Logger 日志框架 log4j2</li>
+ *     <li>org.apache.log4j.Logger 日志框架 log4j </li>
+ *     <li>java.util.logging.Logger 日志框架,JDK的logger</li>
+ *
+ * </ol>
+ * @author Clinton Begin
+ * @author Eduardo Macarron
+ */
+public final class LogFactory {
+
+    /**
+     * Marker to be used by logging implementations that support markers.
+     */
+    public static final String MARKER = "MYBATIS";
+
+    private static Constructor<? extends Log> logConstructor;
+
+    /**
+     * 日志的实现类的具体选择
+     */
+    static {
+        // slf4j 日志
+        tryImplementation(LogFactory::useSlf4jLogging);
+        // apache 日志
+        tryImplementation(LogFactory::useCommonsLogging);
+        // log4j2 日志
+        tryImplementation(LogFactory::useLog4J2Logging);
+        // log4 日志
+        tryImplementation(LogFactory::useLog4JLogging);
+        // JDK 日志
+        tryImplementation(LogFactory::useJdkLogging);
+        // 空 日志
+        tryImplementation(LogFactory::useNoLogging);
+    }
+
+    /**
+     * 私有化构造方法,这是一个单例
+     */
+    private LogFactory() {
+        // disable construction
+    }
+
+    public static Log getLog(Class<?> aClass) {
+        return getLog(aClass.getName());
+    }
+
+    public static Log getLog(String logger) {
+        try {
+            return logConstructor.newInstance(logger);
+        } catch (Throwable t) {
+            throw new LogException("Error creating logger for logger " + logger + ".  Cause: " + t, t);
+        }
+    }
+
+    public static synchronized void useCustomLogging(Class<? extends Log> clazz) {
+        setImplementation(clazz);
+    }
+
+    public static synchronized void useSlf4jLogging() {
+        setImplementation(org.apache.ibatis.logging.slf4j.Slf4jImpl.class);
+    }
+
+    public static synchronized void useCommonsLogging() {
+        setImplementation(org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl.class);
+    }
+
+    public static synchronized void useLog4JLogging() {
+        setImplementation(org.apache.ibatis.logging.log4j.Log4jImpl.class);
+    }
+
+    public static synchronized void useLog4J2Logging() {
+        setImplementation(org.apache.ibatis.logging.log4j2.Log4j2Impl.class);
+    }
+
+    public static synchronized void useJdkLogging() {
+        setImplementation(org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl.class);
+    }
+
+    public static synchronized void useStdOutLogging() {
+        setImplementation(org.apache.ibatis.logging.stdout.StdOutImpl.class);
+    }
+
+    public static synchronized void useNoLogging() {
+        setImplementation(org.apache.ibatis.logging.nologging.NoLoggingImpl.class);
+    }
+
+    /**
+     * 选择具体的日志实现
+     */
+    private static void tryImplementation(Runnable runnable) {
+        if (logConstructor == null) {
+            try {
+                // run()? 似乎违背了代码的语义, 看静态方法.静态方法多行同类型的操作我认为是一个多线程
+                runnable.run();
+            } catch (Throwable t) {
+                // ignore
+            }
+        }
+    }
+
+    /**
+     * 选择具体的日志实现
+     */
+    private static void setImplementation(Class<? extends Log> implClass) {
+        try {
+            Constructor<? extends Log> candidate = implClass.getConstructor(String.class);
+            Log log = candidate.newInstance(LogFactory.class.getName());
+            if (log.isDebugEnabled()) {
+                log.debug("Logging initialized using '" + implClass + "' adapter.");
+            }
+            logConstructor = candidate;
+        } catch (Throwable t) {
+            throw new LogException("Error setting Log implementation.  Cause: " + t, t);
+        }
+    }
+
+}
+
+```
+
+- `LogFactory`是一个单例对象,对外公开`getLog`方法在使用时直接`private static final Log log = LogFactory.getLog(CglibProxyFactory.class);`即可
+
+- 在 `org.apache.ibatis.session.Configuration` 中可以看到下面这些注册方法
+```java
+        // 日志实现类
+        typeAliasRegistry.registerAlias("SLF4J", Slf4jImpl.class);
+        typeAliasRegistry.registerAlias("COMMONS_LOGGING", JakartaCommonsLoggingImpl.class);
+        typeAliasRegistry.registerAlias("LOG4J", Log4jImpl.class);
+        typeAliasRegistry.registerAlias("LOG4J2", Log4j2Impl.class);
+        typeAliasRegistry.registerAlias("JDK_LOGGING", Jdk14LoggingImpl.class);
+        typeAliasRegistry.registerAlias("STDOUT_LOGGING", StdOutImpl.class);
+        typeAliasRegistry.registerAlias("NO_LOGGING", NoLoggingImpl.class);
+
+```