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
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
/* 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
/* 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
/* 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)。
