MyBatis中的日志模块源码分析

MyBatis中的日志模块源码分析

概述

MyBatis没有提供日志的实现类，需要接入第三方的日志组件，但第三方的日志组件有各自的log级别，各不相同，MyBatis统一提供了trace、debug、warn、error四个级别。

概览

MyBatis源码依赖的开源日志组件有：

<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-apiartifactId>
    <version>1.7.30version>
    <optional>trueoptional>
dependency>
<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-log4j12artifactId>
    <version>1.7.30version>
    <optional>trueoptional>
dependency>
<dependency>
    <groupId>log4jgroupId>
    <artifactId>log4jartifactId>
    <version>1.2.17version>
    <optional>trueoptional>
dependency>
<dependency>
    <groupId>org.apache.logging.log4jgroupId>
    <artifactId>log4j-coreartifactId>
    <version>2.13.0version>
    <optional>trueoptional>
dependency>
<dependency>
    <groupId>commons-logginggroupId>
    <artifactId>commons-loggingartifactId>
    <version>1.2version>
    <optional>trueoptional>
dependency>

optional均为true，让使用者可选择哪一种依赖。

Mybatis配置文件中配置使用哪一种日志的地方是：

<settings>
    <setting name="logImpl" value="SLF4J"/>
settings>

可选项共有：SLF4J | LOG4J | LOG4J2 | JDK_LOGGING | COMMONS_LOGGING | STDOUT_LOGGING | NO_LOGGING

加载过程

XMLConfigBuilder的loadCustomLogImpl方法加载MyBatis配置文件的时候，读取logImpl配置的value：

private void loadCustomLogImpl(Properties props) {
    Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
    configuration.setLogImpl(logImpl);
}

在resolveClass方法中，MyBatis的保存了一系列的别名映射，其中就有：

slf4j -> {Class@2218} "class org.apache.ibatis.logging.slf4j.Slf4jImpl"
log4j -> {Class@2265} "class org.apache.ibatis.logging.log4j.Log4jImpl"
log4j2 -> {Class@2279} "class org.apache.ibatis.logging.log4j2.Log4j2Impl"
jdk_logging -> {Class@2267} "class org.apache.ibatis.logging.jdk14.Jdk14LoggingImpl"
commons_logging -> {Class@2259} "class org.apache.ibatis.logging.commons.JakartaCommonsLoggingImpl"
stdout_logging -> {Class@2284} "class org.apache.ibatis.logging.stdout.StdOutImpl"
no_logging -> {Class@2249} "class org.apache.ibatis.logging.nologging.NoLoggingImpl"

然后找到xml中配置的日志别名（解析过程会将配置文件中的日志别名转换成小写）对应的class。

例如我们在配置文件中配置

<settings>
    <setting name="logImpl" value="SLF4J"/>
settings>

效果相同。

找到slf4j对应的class之后，通过Resource类初始化org.apache.ibatis.logging.slf4j.Slf4jImpl类。

Resources.classForName(org.apache.ibatis.logging.slf4j.Slf4jImpl)

然后将org.apache.ibatis.logging.slf4j.Slf4jImpl赋予Configuration类的logImpl属性。

Configuration尤其重要，基本所有重要的MyBatis配置都在里面。

Configuration.setLogImpl()方法如下：

public void setLogImpl(Class<? extends Log> logImpl) {
    if (logImpl != null) {
        this.logImpl = logImpl;
        LogFactory.useCustomLogging(this.logImpl);
    }
}

在进入LogFactory.useCustomLogging之前，先看MyBatis的日志接口和实现类(Slf4jImpl为例)：

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);

}

public class Slf4jImpl implements Log {

  private Log log;

  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);
  }

}

其他日志组件log4j等的实现类类似。

继续回到LogFactory.useCustomLogging方法中：

public final class LogFactory {

  public static final String MARKER = "MYBATIS";

  private static Constructor<? extends Log> logConstructor;

  static {
    //MyBatis对于第三方日志加载的优先级是：slf4j->commonsloging->log4j2->log4j->jdklog
    tryImplementation(LogFactory::useSlf4jLogging);
    tryImplementation(LogFactory::useCommonsLogging);
    tryImplementation(LogFactory::useLog4J2Logging);
    tryImplementation(LogFactory::useLog4JLogging);
    tryImplementation(LogFactory::useJdkLogging);
    tryImplementation(LogFactory::useNoLogging);
  }

  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);
  }

  private static void tryImplementation(Runnable runnable) {
    if (logConstructor == null) {
      try {
        runnable.run();
      } catch (Throwable t) {
        // ignore
      }
    }
  }

  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);
    }
  }

}

根据class的类型，通过反射，获取构造方法对象Constructor，并使用构造方法（传一个String参数，值就是日志的class名称）初始化日志类，尝试着输出测试一下日志（debug级别）。

然后将构造方法对象赋予logConstructor变量。然后通过下面的方法在MyBatis源码各个地方被四处调用：

public static Log getLog(String logger) {
    return logConstructor.newInstance(logger);
}

其中就包括LoggingCache日志缓存类。二级缓存命中率就是在此类中输出的。

public class LoggingCache implements Cache {

  private final Log log;
  private final Cache delegate;
  protected int requests = 0;
  protected int hits = 0;

  public LoggingCache(Cache delegate) {
    this.delegate = delegate;
    this.log = LogFactory.getLog(getId());
  }

  @Override
  public String getId() {
    return delegate.getId();
  }

  @Override
  public int getSize() {
    return delegate.getSize();
  }

  @Override
  public void putObject(Object key, Object object) {
    delegate.putObject(key, object);
  }

  @Override
  public Object getObject(Object key) {
    requests++;
    final Object value = delegate.getObject(key);
    if (value != null) {
      hits++;
    }
    if (log.isDebugEnabled()) {
      log.debug("Cache Hit Ratio [" + getId() + "]: " + getHitRatio());
    }
    return value;
  }

  @Override
  public Object removeObject(Object key) {
    return delegate.removeObject(key);
  }

  @Override
  public void clear() {
    delegate.clear();
  }

  @Override
  public int hashCode() {
    return delegate.hashCode();
  }

  @Override
  public boolean equals(Object obj) {
    return delegate.equals(obj);
  }

  private double getHitRatio() {
    return (double) hits / (double) requests;
  }

}

由于用到了装饰模式，下面代码中的TransactionalCache二级缓存对象，它有一个属性是Cache，实际运行的真身就是LoggingCache。

/**
 * The 2nd level cache transactional buffer.
 * 

 * This class holds all cache entries that are to be added to the 2nd level cache during a Session.
 * Entries are sent to the cache when commit is called or discarded if the Session is rolled back.
 * Blocking cache support has been added. Therefore any get() that returns a cache miss
 * will be followed by a put() so any lock associated with the key can be released.
 */
public class TransactionalCache implements Cache {
  private final Cache delegate;
  
  @Override
  public Object getObject(Object key) {
    // issue #116
    Object object = delegate.getObject(key);
    if (object == null) {
      entriesMissedInCache.add(key);
    }
    // issue #146
    if (clearOnCommit) {
      return null;
    } else {
      return object;
    }
  }

查询缓存的时候，如果开启了二级缓存，通过装饰模式间接调用LoggingCache统计命中率。命中率即hits次数/请求的次数。

适配器模式

https://blog.csdn.net/smith789/article/details/104207786

MyBatis日志模块用到了适配器模式。例如：

class Slf4jLoggerImpl implements Log {  
    private final Logger log;

Slf4jLoggerImpl是针对slf4j的适配器，log变量是真正提供日志能力的类。

日志模块类图