插件使用

动手实现plugin

首先我们需要实现一下这个Interceptor，其中plugin和setProperties方法可以不实现，plugin是因为已经有了完善的逻辑，而setProperties，如果不需要在intercept()中使用属性，也可以不设置。然后在实现类中完成注解配置

public interface Interceptor {Object intercept(Invocation invocation) throws Throwable;default Object plugin(Object target) {return Plugin.wrap(target, this);}default void setProperties(Properties properties) {// NOP}}

以下是一个实现demo

@Intercepts({@Signature(type = StatementHandler.class,method = "prepare",args = {Connection.class, Integer.class}
), @Signature(type = Executor.class,method = "update",args = {MappedStatement.class, Object.class}
), @Signature(type = Executor.class,method = "query",args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}
), @Signature(type = Executor.class,method = "query",args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}
)})
public class testMybatisInterceptor implements Interceptor {private Properties properties;@Overridepublic Object intercept(Invocation invocation) throws Throwable {return invocation.proceed();}@Overridepublic Object plugin(Object target) {return Plugin.wrap(target, this);}@Overridepublic void setProperties(Properties properties) {this.properties = properties;}public Properties getProperties() {return properties;}}

向容器注入该interceptor

可以通过两种方式
一种是@Bean注入

@Bean
public MybatisPlusInterceptor mybatisPlusInterceptor() {return new testMybatisInterceptor();
}

另一种是xml注入，目前比较少用了

<plugins><plugin interceptor="com.testMybatisInterceptor"></plugin>
</plugins>

拦截器原理

public class InterceptorChain {private final List<Interceptor> interceptors = new ArrayList<>();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);}}

拦截器的添加

初始化的添加

Configuration是mybatis的核心初始化数据承装容器，在一开始它就会创建一个InterceptorChain，后期所有添加和过滤都是在这个InterceptorChain上操作的。

public class InterceptorChain {private final List<Interceptor> interceptors = new ArrayList<>();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);}}

@Bean

@Bean只是本人的猜测，mybatis应该会添加一个beanPostProcessor，对于当前bean是否拥有@interceptors注解进行扫描，如果有，就会把这个bean放入Configuration

XML

在解析XML的时候，就会把拦截器添加进configuration

  private void pluginElement(XNode parent) throws Exception {if (parent != null) {for (XNode child : parent.getChildren()) {String interceptor = child.getStringAttribute("interceptor");Properties properties = child.getChildrenAsProperties();Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();interceptorInstance.setProperties(properties);configuration.addInterceptor(interceptorInstance);}}}

初始化Handler时的设置

在完成newParameter的时候，就会对Handler进行拦截器封装。封装的过程就是使用代理模式，invokeHandler就是

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject,BoundSql boundSql) {ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement,parameterObject, boundSql);return (ParameterHandler) interceptorChain.pluginAll(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);return (ResultSetHandler) interceptorChain.pluginAll(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);return (StatementHandler) interceptorChain.pluginAll(statementHandler);}public Executor newExecutor(Transaction transaction, ExecutorType executorType) {executorType = executorType == null ? defaultExecutorType : 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);}return (Executor) interceptorChain.pluginAll(executor);}

在pluginAll中，会对每一个Interceptor进行接口拦截

  public Object pluginAll(Object target) {for (Interceptor interceptor : interceptors) {target = interceptor.plugin(target);}return target;}

target就是以上某种Handler或者Executor，interceptor就是当前遍历的执行器

  public static Object wrap(Object target, Interceptor interceptor) {Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);Class<?> type = target.getClass();Class<?>[] interfaces = getAllInterfaces(type, signatureMap);if (interfaces.length > 0) {return Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap));}return target;}

其中getSignatureMap，就是把注解转化为map，供以后调用使用，里面的key就是type，而value就是prepare(Connection,Integer)这个方法。

@Intercepts({@Signature(type = StatementHandler.class,method = "prepare",args = {Connection.class, Integer.class}
)})

在invoke方法中，会先检查这个方法是否通过注解注册过，如果是注册的方法，就会调用intercept方法，并且把当前handler，当前方法，和方法参数返回给interceptor完成后续业务操作。

  @Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {Set<Method> methods = signatureMap.get(method.getDeclaringClass());if (methods != null && methods.contains(method)) {return interceptor.intercept(new Invocation(target, method, args));}return method.invoke(target, args);} catch (Exception e) {throw ExceptionUtil.unwrapThrowable(e);}}

处理拦截器

对应Handler在调用方法时，会调用intercept()，如果通过，会调用method.invoke()进行真正的代码执行，如果在拦截器模式下，由于是代理模式套代理模式层层循环，所以实际上是调用了第二个代理器的intercept()，直到完全调用完，才会进入最内层的被代理对象的源方法。

@Override
public Object intercept(Invocation invocation) throws Throwable {return invocation.proceed();
}

public Object proceed() throws InvocationTargetException, IllegalAccessException {return method.invoke(target, args);
}