mybatis之插件机制

插件

使用

1. 使用插件意味着在修改mybatis的底层封装，虽然灵活但是也可能导致mybatis出现重大bug

2. 在mybatis中使用插件就必须实现Interceptor接口

   public interface Interceptor {
   
     //直接覆盖所拦截对象原有的方法，通过Invocation反射调用原来对象的方法
     Object intercept(Invocation invocation) throws Throwable;
   // target指的是被拦截的对象，对拦截对象生成一个代理对象，并返回
     Object plugin(Object target);
   //在plugin元素中配置所需要的参数，在插件初始化的时候调用一次
     void setProperties(Properties properties);
   
   }
   
   public class InterceptorChain {
   
     private final List 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);
     }
   
   }
   

3. mybatis提供了一个工具类Plugin，实现了InvocationHandler接口，采用jdk动态代理

   public class Plugin implements InvocationHandler {
   
     private Object target;
     private Interceptor interceptor;
     private Map, Set> signatureMap;
   
     private Plugin(Object target, Interceptor interceptor, Map, Set> signatureMap) {
       this.target = target;
       this.interceptor = interceptor;
       this.signatureMap = signatureMap;
     }
   
     public static Object wrap(Object target, Interceptor interceptor) {
       Map, Set> 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;
     }
   
     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
       try {
         Set 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);
       }
     }
   
     private static Map, Set> getSignatureMap(Interceptor interceptor) {
       Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
       if (interceptsAnnotation == null) { // issue #251
         throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
       }
       Signature[] sigs = interceptsAnnotation.value();
       Map, Set> signatureMap = new HashMap, Set>();
       for (Signature sig : sigs) {
         Set methods = signatureMap.get(sig.type());
         if (methods == null) {
           methods = new HashSet();
           signatureMap.put(sig.type(), methods);
         }
         try {
           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;
     }
   
     private static Class[] getAllInterfaces(Class type, Map, Set> signatureMap) {
       Set> interfaces = new HashSet>();
       while (type != null) {
         for (Class c : type.getInterfaces()) {
           if (signatureMap.containsKey(c)) {
             interfaces.add(c);
           }
         }
         type = type.getSuperclass();
       }
       return interfaces.toArray(new Class[interfaces.size()]);
     }
   
   }

确定需要拦截的签名

1. Mybatis插件可以拦截Executor、StatementHandler、ParameterHandler、ResulteSetHandler中的任意一个。
   * Executor是执行SQL的全过程，包括组装参数，组装结果集返回和执行SQL过程
   * StatementHandler是执行SQL的过程，可以重写执行SQL的过程，这是常用的拦截对象
   * ParameterHandler，很明显它主要是拦截执行SQL的参数组装，你可以重写组装参数规则
   * ResultSetHandler用于拦截执行结果的组装，你可以重写组装结果的规则

2. 案例

       配置插件，注意顺序
           * Mybatis3.X 的版本使用的 dtd 作为 XML 的格式校验文档。
           * 而在 XML 规范中，dtd 是有严格的顺序的，在报错的异常中已经列出了对应的顺序，
           * 应该为：(properties?,settings?,typeAliases?,typeHandlers?,objectFactory?,
           objectWrapperFactory?,plugins?,environments?,databaseIdProvider?,mappers?)
   
       
           "com.jannal.mybatis.plugin.MyPlugin">
               "jannal" value="jannalpro" />
           
       
   
        /**
    * @Intercepts 标识它是一个拦截器
    * @Signature 注册拦截器签名的地方
    */
   @Intercepts(value = { @Signature(type = StatementHandler.class, // 确定要拦截的对象
           method = "prepare", // 确定要拦截的方法
           args = { Connection.class }// 拦截方法的参数
           ) })
   public class MyPlugin implements Interceptor {
       private Properties properties;
   
       @Override
       public Object intercept(Invocation invocation) throws Throwable {
           System.out.println("开始拦截.....");
           Object proceed = invocation.proceed();
           System.out.println("结束拦截.....");
           return proceed;
       }
   
       @Override
       public Object plugin(Object target) {
           System.out.println("生成代理对象....");
           return Plugin.wrap(target, this);
       }
   
       @Override
       public void setProperties(Properties properties) {
           System.out.println(properties.get("jannal"));
          this.properties = properties;
       }
   
   }
   
   生成代理对象....
   生成代理对象....
   生成代理对象....
   生成代理对象....
   2017-06-23 11:50:50.790 [main] DEBUG org.apache.ibatis.logging.slf4j.Slf4jImpl.debug(Slf4jImpl.java:43) - Openning JDBC Connection
   2017-06-23 11:50:50.998 [main] DEBUG org.apache.ibatis.logging.slf4j.Slf4jImpl.debug(Slf4jImpl.java:43) - Created connection 982757413.
   2017-06-23 11:50:51.001 [main] DEBUG org.apache.ibatis.logging.slf4j.Slf4jImpl.debug(Slf4jImpl.java:43) - ooo Using Connection [com.mysql.jdbc.JDBC4Connection@3a93b025]
   开始拦截.....
   2017-06-23 11:50:51.002 [main] DEBUG org.apache.ibatis.logging.slf4j.Slf4jImpl.debug(Slf4jImpl.java:43) - ==>  Preparing: select id ,mobile_no_internet ,cust_no ,mes_code ,mes_title ,mes_content ,create_time ,inserttime ,updatetime from t_user_message t where t.id=?; 
   结束拦截.....