mybatis本身沒有提供日志的實現，引入的是第三方組件。mybatis支持多個第三方日志插件，優先級由低到高為slf4J、commonsLoging、Log4J2、Log4J和JdkLog。
mybatis中有一個LogFactory，獲取log的工廠類，在工程類中可以回去對應的日志實現。分析工程類，可以發現mybatis如何來選擇log
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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); }} |
關于logConstructor的加載如下
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static { tryImplementation(LogFactory::useSlf4jLogging); tryImplementation(LogFactory::useCommonsLogging); tryImplementation(LogFactory::useLog4J2Logging); tryImplementation(LogFactory::useLog4JLogging); tryImplementation(LogFactory::useJdkLogging); tryImplementation(LogFactory::useNoLogging);} |
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private static void tryImplementation(Runnable runnable) { if (logConstructor == null) { try { runnable.run(); } catch (Throwable t) { // ignore } }} |
在 tryImplementation ，中會設置mybatis使用的log類型。把引用的log設置到logConstructor中后，后續其他類型的log也不會再加載。所以在mybatis中優先級由低到高為slf4J、commonsLoging、Log4J2、Log4J和JdkLog。感覺也是屬于SPI的一種實現方式，不同的是各種類型的第三方日志，無法形成一個統一的接口。故此，mybatis為了解決這一問題，使用了適配器模式。
適配器的實現一般是讓適配器實現或者繼承目標，并且內部持有一個適配者的引用。這樣調用目標對象方法，實際上是調用適配者的方法。
mybatis 又是如何把這log，用起來的。根據mybatis的習慣，應該會使用代理模式，來打印這個日志。 舉例查詢的語句查看，根據MapperProxy,查到最后查詢的語句
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public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { Statement stmt = null; try { Configuration configuration = ms.getConfiguration(); StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); stmt = prepareStatement(handler, ms.getStatementLog()); return handler.query(stmt, resultHandler); } finally { closeStatement(stmt); }} |
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private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; Connection connection = getConnection(statementLog); stmt = handler.prepare(connection, transaction.getTimeout()); handler.parameterize(stmt); return stmt;} |
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protected Connection getConnection(Log statementLog) throws SQLException { Connection connection = transaction.getConnection(); if (statementLog.isDebugEnabled()) { return ConnectionLogger.newInstance(connection, statementLog, queryStack); } else { return connection; }} |
到此處可以看到mybatis在獲取連接的時候，會根據日志的打印級別來判斷是否會創建一個代理類。到這里就基本可以猜到，在代理類中，mybatis會去打印這個sql的語句
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public static Connection newInstance(Connection conn, Log statementLog, int queryStack) { InvocationHandler handler = new ConnectionLogger(conn, statementLog, queryStack); ClassLoader cl = Connection.class.getClassLoader(); return (Connection) Proxy.newProxyInstance(cl, new Class[]{Connection.class}, handler);} |
用 ConnectionLogger 來舉例，看到里面的invoke的方法
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public Object invoke(Object proxy, Method method, Object[] params) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, params); } if ("prepareStatement".equals(method.getName())) { if (isDebugEnabled()) { debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true); } PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params); stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack); return stmt; } else if ("prepareCall".equals(method.getName())) { if (isDebugEnabled()) { debug(" Preparing: " + removeBreakingWhitespace((String) params[0]), true); } PreparedStatement stmt = (PreparedStatement) method.invoke(connection, params); stmt = PreparedStatementLogger.newInstance(stmt, statementLog, queryStack); return stmt; } else if ("createStatement".equals(method.getName())) { Statement stmt = (Statement) method.invoke(connection, params); stmt = StatementLogger.newInstance(stmt, statementLog, queryStack); return stmt; } else { return method.invoke(connection, params); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); }} |
可以看到，mybatis在里面還可以更具情況創建代理類。代理類又一次被代理，這也是mybatis喜歡的編程方式，比如插件也是代理類再次被代理，來實現多個插件并行。
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原文鏈接：https://juejin.im/post/5f0dbfed6fb9a07ec07b4888
