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MyBatis之plugin原理及使用

MyBatis之plugin原理及使用

作者: 寒冬的骄阳 | 来源:发表于2018-12-09 10:45 被阅读0次

    实现原理

    首先Plugin必须实现Interceptor 常拦截以下类或接口中的方法:

    • Executor
    • ParameterHandler
    • ResultSetHandler
    • StatementHandler
      与Plugin有关的类:pluginInterceptorChain
    @Intercepts({@Signature(type = Executor.class,
            method ="query",
            args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
    public class CustomPlugins implements Interceptor {
        @Override
        public Object intercept(Invocation invocation) throws Throwable {
            //最终plugin插件调用的是这个方法
            MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
            BoundSql boundSql = mappedStatement.getBoundSql(invocation.getArgs()[1]);
            System.out.println(String.format("plugin output sql = %s , param=%s", boundSql.getSql(),boundSql.getParameterObject()));
            //放行 该方法
            return invocation.proceed();
        }
    
        @Override
        public Object plugin(Object o) {
            Object obj= Plugin.wrap(o,this);
            return obj;
        }
    
        @Override
        public void setProperties(Properties properties) {
            //常用于将配置中的参数赋值给类的实例变量
            String value = (String) properties.get("name");
            System.out.println(value);
        }
    }
    
    //InterceptorChain
    public class InterceptorChain {
    
      private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
    
      public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {
          //这里的intecetor是Plugin 也即是自定义插件中plugin方法 
          //这里的target是Executor、ParameterHandler、ResultSetHandler、StatementHandler接口的实现类 具体是什么 要看@Interceptors中拦截的接口
          //这里的target是通过for循环不断赋值的,也就是说如果有多个拦截器,那么如果我用P表示代理,生成第       //一次代理为P(target),生成第二次代理为P(P(target)),生成第三次代理为P(P(P(target))),不断      //嵌套下去,这就得到一个重要的结论:<plugins>...</plugins>中后定义的<plugin>实际其拦截器方法     //先被执行,因为根据这段代码来看,后定义的<plugin>代理实际后生成,包装了先生成的代理,自然其代理方     //法也先执行.也即是interceptor的执行顺序 后定义先执行
          //这里的plugin方法实际上是调用对应拦截器类的重载方法
          target = interceptor.plugin(target);
        }
        return target;
      }
    
      public void addInterceptor(Interceptor interceptor) {
          //添加拦截器至集合
        interceptors.add(interceptor);
      }
     // 列举所有的拦截器类 
      public List<Interceptor> getInterceptors() {
        return Collections.unmodifiableList(interceptors);
      }
    
    }
    
    //Plugin
    public static Object wrap(Object target, Interceptor interceptor) {
        //获取拦截器的签名信息,具体代码如下
        Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
        Class<?> type = target.getClass();
        Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
        //如果数组长度大于0 也就是表示type有父接口 可以根据其接口生成代理对象
        if (interfaces.length > 0) {
          return Proxy.newProxyInstance(
              type.getClassLoader(),
              interfaces,
              new Plugin(target, interceptor, signatureMap));
        }
        //否则 返回target对象本身
        return target;
      }
    
    @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            //method#getDeclaringClass() 获取method对应的class类 
            //以CustomPlugin为例,此时method为query 得到的的declaringClass为Executor
            //此时签名Map中可以得到key为Executor的value
          Set<Method> methods = signatureMap.get(method.getDeclaringClass());
          if (methods != null && methods.contains(method)) {
              //这里调用拦截器中的intercept方法 拦截器对应具体的实现 如此时是CustomPlugin
            return interceptor.intercept(new Invocation(target, method, args));
          }
          return method.invoke(target, args);
        } catch (Exception e) {
          throw ExceptionUtil.unwrapThrowable(e);
        }
      }
    
    //getSignatureMap方法
    
      private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
        Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
        // issue #251
          //如果拦截器类没有Intercepts注解则抛出异常
        if (interceptsAnnotation == null) {
          throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
        }
          //以CustomPlugins为例@Intercepts({@Signature(type = Executor.class,
          //method ="query",
        //args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
        //type=Executor.class,method=query args=...
        Signature[] sigs = interceptsAnnotation.value();
          //如果存在以type为key的map 则获取其拦截的方法 否则执行map的put方法 为key赋值
        Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
        for (Signature sig : sigs) {
          Set<Method> methods = signatureMap.get(sig.type());
          if (methods == null) {
            methods = new HashSet<Method>();
            signatureMap.put(sig.type(), methods);
          }
          try {
              //利用反射 获取Executor中方法为query,参数为sig.args的方法
            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);
          }
        }
        //返回签名map
        return signatureMap;
      }
    //getInterfaces
    private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
        Set<Class<?>> interfaces = new HashSet<Class<?>>();
        while (type != null) {
            //如此时type为Executor的具体实例 CachingExecutor
            //type.getInterfaces 得到的是Executor
            //刚好Executor.class 为map的key
          for (Class<?> c : type.getInterfaces()) {
            if (signatureMap.containsKey(c)) {
              interfaces.add(c);
            }
          }
            //向上查询 查询其父类
          type = type.getSuperclass();
        }
        return interfaces.toArray(new Class<?>[interfaces.size()]);
      }
    

    使用方式

    自定义插件常通过实现Interceptor接口的方式实现

    //表示拦截的是哪个类的含有几个参数的方法
    @Intercepts({@Signature(type = Executor.class,
            method ="query",
            args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
    public class CustomPlugins implements Interceptor {
        @Override
        public Object intercept(Invocation invocation) throws Throwable {
            //一系列的操作 然后拦截器放行
            return invocation.proceed();
        }
    
        @Override
    public Object plugin(Object o) {
       //o表示拦截接口的子类 this表示CustomPlugins
            return Plugin.wrap(o,this);
        }
    
        @Override
        public void setProperties(Properties properties) {
         //读取配置插件时 给出的参数信息
     //常用于将配置中的参数赋值给类的实例变量
            String value = (String) properties.get("name");
            System.out.println(value);
        }
    }
    

    配置plugin的两种形式:

    • 基于xml的形式
    //mybatis.xml
    <plugins>
      <plugin interceptor="com.wojiushiwo.dal.plugins.CustomPlugins">
        <property name="name" value="wojiushiwo"/>
      </plugin>
    </plugins>
    
    • 基于Java config的形式
    //使用的时候直接在注入即可
    @Bean
    public SqlSessionFactory localSessionFactoryBean() throws Exception {
            SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
            sqlSessionFactoryBean.setDataSource(dataSource);
            //将自定义的typeHandler设置到mybatis配置文件中
            sqlSessionFactoryBean.setTypeHandlers(new TypeHandler[]{new CustomTypeHandler()});
            //将自定义的插件设置到mybatis配置中
            sqlSessionFactoryBean.setPlugins(new Interceptor[]{plugin()});
            SqlSessionFactory factory = sqlSessionFactoryBean.getObject();
            factory.getConfiguration().setLazyLoadingEnabled(true);
            factory.getConfiguration().setAggressiveLazyLoading(false);
            factory.getConfiguration().setProxyFactory(new CglibProxyFactory());
            return factory;
        }
    
        private Interceptor plugin(){
            CustomPlugins customPlugins=new CustomPlugins();
            Properties properties=new Properties();
            //这里可以设置自定义参数
            properties.setProperty("name","wojiushiwo");
            customPlugins.setProperties(properties);
            return customPlugins;
        }
    

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