有许多java初学者对于MyBatis拦截器Inteceptor不是很了解，在这里我来为各位整理下篇关于java中MyBatis拦截器Inteceptor详解，
本文主要分析MyBatis的插件机制，实际就是Java动态代理实现的责任链模式实现。
根据官方文档。Mybatis只允许拦截以下方法，这个决定写拦截器注解签名参数。
代码如下
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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) |
拦截处理的源码如下，其中interceptorChain.pluginAll(..)即为织入自定义拦截器：
代码如下
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/* org.apache.ibatis.session.Configuration类中方法 */public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); /* 拦截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); /* 拦截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); /* 拦截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); } /* 拦截Executor*/ executor = (Executor) interceptorChain.pluginAll(executor); return executor;} |
实现一个自定义拦截器只需实现Interceptor接口即可，大致代码如下:
代码如下
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/* 注解表明要拦截哪个接口的方法及其参数 */@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class }) })public class YourInterceptor implements Interceptor{ public Object intercept(Invocation invocation) throws Throwable{ doSomeThing(); /* 注：此处实际上使用Invocation.proceed()方法完成interceptorChain链的遍历调用(即执行所有注册的Interceptor的intercept方法)，到最终被代理对象的原始方法调用 */ return invocation.proceed(); } /*生成成对目标target的代理，而@Intercepts的注解是在Plugin.wrap中用到*/ @Override public Object plugin(Object target){ /* 当目标类是StatementHandler类型时，才包装目标类，不做无意义的代理 */ return (target instanceof StatementHandler)?Plugin.wrap(target, this):target; } /*用于设置自定义的拦截器配置参数*/ @Override public void setProperties(Properties properties){ }} |
其中，拦截调用的代码均在Plugin.wrap中：
代码如下
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/* org.apache.ibatis.plugin.Plugin类 */public class Plugin implements InvocationHandler { /* 省略代码... */ public static Object wrap(Object target, Interceptor interceptor) { /* 此处即为获取Interceptor的注解签名 */ Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor); Class<?> type = target.getClass(); /* 获取拦截目标类相匹配的接口 */ Class<?>[] interfaces = getAllInterfaces(type, signatureMap); if (interfaces.length > 0) { /* 使用jdk动态代理 */ return Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap)); } return target; } /* 拦截目标类的所有方法的执行都会变为在此执行 */ @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)) { /* 执行拦截器方法 */ return interceptor.intercept(new Invocation(target, method, args)); } return method.invoke(target, args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } } /* 省略代码... */} |
可以看到MyBatis的拦截器设计核心代码还是比较简单的，但是足够灵活。实际使用时注意，不做无意义的代理(Plugin.wrap)。
