Mybatis插件原理分析（一）

/** * @author Clinton Begin */ public interface Interceptor { //在此方法中实现自己需要的功能，最后履行invocation.proceed()方法，实际就是调用method.invoke(target,args)方法，调用代理类 Object intercept(Invocation invocation) throws Throwable; //这个方法是将target生成代理类 Object plugin(Object target); //在xml中注册Intercept是配置1些属性 void setProperties(Properties properties); }

/** * @author Clinton Begin */ public class InterceptorChain { //插件拦截器链 private final List<Interceptor> interceptors = new ArrayList<Interceptor>(); //把target变成代理类，这样在运行target方法之前需要运行Plugin的invoke方法 public Object pluginAll(Object target) { for (Interceptor interceptor : interceptors) { target = interceptor.plugin(target); } return target; } public void addInterceptor(Interceptor interceptor) { interceptors.add(interceptor); } public List<Interceptor> getInterceptors() { return Collections.unmodifiableList(interceptors); } }

/** * @author Clinton Begin */ public class Invocation { private Object target; private Method method; private Object[] args; public Invocation(Object target,Method method,Object[] args) { this.target = target; this.method = method; this.args = args; } public Object getTarget() { return target; } public Method getMethod() { return method; } public Object[] getArgs() { return args; } //其实mybatis的Interceptor终究还是调用的method.invoke方法 public Object proceed() throws InvocationTargetException,IllegalAccessException { return method.invoke(target,args); } }

/** * @author Clinton Begin */ //这个类是Mybatis拦截器的核心,大家可以看到该类继承了InvocationHandler //又是JDK动态代理机制 public class Plugin implements InvocationHandler { //目标对象 private Object target; //拦截器 private Interceptor interceptor; //记录需要被拦截的类与方法 private Map<Class<?>,Set<Method>> signatureMap; private Plugin(Object target,Interceptor interceptor,Map<Class<?>,Set<Method>> signatureMap) { this.target = target; this.interceptor = interceptor; this.signatureMap = signatureMap; } //1个静态方法,对1个目标对象进行包装，生成代理类。 public static Object wrap(Object target,Interceptor interceptor) { //首先根据interceptor上面定义的注解 获得需要拦截的信息 Map<Class<?>,Set<Method>> signatureMap = getSignatureMap(interceptor); Class<?> type = target.getClass(); Class<?>[] interfaces = getAllInterfaces(type,signatureMap); //如果长度为>0 则返回代理类 否则不做处理 if (interfaces.length > 0) { //创建JDK动态代理对象 return Proxy.newProxyInstance( type.getClassLoader(),interfaces,new Plugin(target,interceptor,signatureMap)); } return target; } //在履行Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类的方法时会调用这个方法 @Override public Object invoke(Object proxy,Object[] args) throws Throwable { try { //通过method参数定义的类 去signatureMap当中查询需要拦截的方法集合 Set<Method> methods = signatureMap.get(method.getDeclaringClass()); //判断是不是是需要拦截的方法，如果需要拦截的话就履行实现的Interceptor的intercept方法，履行完以后还是会履行method.invoke方法，不过是放到interceptor实现类中去实现了 if (methods != null && methods.contains(method)) { return interceptor.intercept(new Invocation(target,method,args)); } //不拦截 直接通过目标对象调用方法 return method.invoke(target,args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } } //根据拦截器接口（Interceptor）实现类上面的注解获得相干信息 private static Map<Class<?>,Set<Method>> getSignatureMap(Interceptor interceptor) { //获得注解信息 Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class); // issue #251 //为空则抛出异常 if (interceptsAnnotation == null) { throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName()); } //取得Signature注解信息 Signature[] sigs = interceptsAnnotation.value(); Map<Class<?>,Set<Method>> signatureMap = new HashMap<Class<?>,Set<Method>>(); //循环注解信息 for (Signature sig : sigs) { //根据Signature注解定义的type信息去signatureMap当中查询需要拦截方法的集合 Set<Method> methods = signatureMap.get(sig.type()); //第1次肯定为null 就创建1个并放入signatureMap if (methods == null) { methods = new HashSet<Method>(); signatureMap.put(sig.type(),methods); } try { //找到sig.type当中定义的方法 并加入到集合 Method method = sig.type().getMethod(sig.method(),sig.args()); methods.add(method); } catch (NoSuchMethodException e) { throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e,e); } } return signatureMap; } //根据对象类型与signatureMap获得接口信息 private static Class<?>[] getAllInterfaces(Class<?> type,Set<Method>> signatureMap) { Set<Class<?>> interfaces = new HashSet<Class<?>>(); //循环type类型的接口信息 如果该类型存在与signatureMap当中则加入到set当中去 while (type != null) { for (Class<?> c : type.getInterfaces()) { if (signatureMap.containsKey(c)) { interfaces.add(c); } } type = type.getSuperclass(); } //转换为数组返回 return interfaces.toArray(new Class<?>[interfaces.size()]); } }

public ParameterHandler newParameterHandler(MappedStatement mappedStatement,Object parameterObject,BoundSql boundSql) { ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement,parameterObject,boundSql); 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); resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler); return resultSetHandler; } public StatementHandler newStatementHandler(Executor executor,BoundSql boundSql) { StatementHandler statementHandler = new RoutingStatementHandler(executor,rowBounds,boundSql); statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler); return statementHandler; } public Executor newExecutor(Transaction transaction) { return newExecutor(transaction,defaultExecutorType); } 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); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; }