有許多java初學者對于MyBatis攔截器Inteceptor不是很了解，在這里我來為各位整理下篇關于java中MyBatis攔截器Inteceptor詳解，
本文主要分析MyBatis的插件機制，實際就是Java動態代理實現的責任鏈模式實現。
根據官方文檔。Mybatis只允許攔截以下方法，這個決定寫攔截器注解簽名參數。
代碼如下
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Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)ParameterHandler (getParameterObject, setParameters)ResultSetHandler (handleResultSets, handleOutputParameters)StatementHandler (prepare, parameterize, batch, update, query) |
攔截處理的源碼如下，其中interceptorChain.pluginAll(..)即為織入自定義攔截器：
代碼如下
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/* org.apache.ibatis.session.Configuration類中方法 */public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); /* 攔截ParameterHandler*/ parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler); return parameterHandler;}public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql) { ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds); /* 攔截ResultSetHandler*/ resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler); return resultSetHandler;}public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); /* 攔截StatementHandler*/ statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler); return statementHandler;}public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { executor = new SimpleExecutor(this, transaction); } if (cacheEnabled) { executor = new CachingExecutor(executor); } /* 攔截Executor*/ executor = (Executor) interceptorChain.pluginAll(executor); return executor;} |
實現一個自定義攔截器只需實現Interceptor接口即可，大致代碼如下:
代碼如下
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/* 注解表明要攔截哪個接口的方法及其參數 */@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class }) })public class YourInterceptor implements Interceptor{ public Object intercept(Invocation invocation) throws Throwable{ doSomeThing(); /* 注：此處實際上使用Invocation.proceed()方法完成interceptorChain鏈的遍歷調用(即執行所有注冊的Interceptor的intercept方法)，到最終被代理對象的原始方法調用 */ return invocation.proceed(); } /*生成成對目標target的代理，而@Intercepts的注解是在Plugin.wrap中用到*/ @Override public Object plugin(Object target){ /* 當目標類是StatementHandler類型時，才包裝目標類，不做無意義的代理 */ return (target instanceof StatementHandler)?Plugin.wrap(target, this):target; } /*用于設置自定義的攔截器配置參數*/ @Override public void setProperties(Properties properties){ }} |
其中，攔截調用的代碼均在Plugin.wrap中：
代碼如下
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/* org.apache.ibatis.plugin.Plugin類 */public class Plugin implements InvocationHandler { /* 省略代碼... */ public static Object wrap(Object target, Interceptor interceptor) { /* 此處即為獲取Interceptor的注解簽名 */ Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor); Class<?> type = target.getClass(); /* 獲取攔截目標類相匹配的接口 */ Class<?>[] interfaces = getAllInterfaces(type, signatureMap); if (interfaces.length > 0) { /* 使用jdk動態代理 */ return Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap)); } return target; } /* 攔截目標類的所有方法的執行都會變為在此執行 */ @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { Set<Method> methods = signatureMap.get(method.getDeclaringClass()); if (methods != null && methods.contains(method)) { /* 執行攔截器方法 */ return interceptor.intercept(new Invocation(target, method, args)); } return method.invoke(target, args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } } /* 省略代碼... */} |
可以看到MyBatis的攔截器設計核心代碼還是比較簡單的，但是足夠靈活。實際使用時注意，不做無意義的代理(Plugin.wrap)。
