mybatis插件

mybatis插件

使用

1. 注册插件

   <plugins>
       <plugin interceptor="com.github.pagehelper.PageInterceptor"></plugin>
   </plugins>
   

2. 编写插件类 实现org.apache.ibatis.plugin.Interceptor接口

3. 添加注解

   @Intercepts(
           {
                   @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 InterceptorTwo implements Interceptor
   

   1. Signature

   @Documented
   @Retention(RetentionPolicy.RUNTIME)
   @Target({})
   public @interface Signature {
     //拦截的方法类型
     Class<?> type();
     //拦截的方法名字
     String method();
     //方法中的参数类型集合
     Class<?>[] args();
   }
   

   2. Intercepts

      @Documented
      @Retention(RetentionPolicy.RUNTIME)
      @Target(ElementType.TYPE)
      public @interface Intercepts {
        //返回需要拦截的方法签名列表
        Signature[] value();
      }
      

插件初始化流程

1. 项目启动加载配置文件

2. 解析配置文件

3. 解析plugins标签

   XMLConfigBuilder

   private void parseConfiguration(XNode root) {
       //解析plus标签
       pluginElement(root.evalNode("plugins"));
   }    
   

4. 循环插入插件类

   XMLConfigBuilder

   //解析拦截器类并插入list集合
   private void pluginElement(XNode parent) throws Exception {
     if (parent != null) {
       //循环获取子类并插入一个List 配置文件中plugin的顺序与执行的顺序相反
       // child = <plugin interceptor="com.github.pagehelper.PageInterceptor"/>
       for (XNode child : parent.getChildren()) {
         //获取interceptor属性
         String interceptor = child.getStringAttribute("interceptor");
         //获取参数
         Properties properties = child.getChildrenAsProperties();
         //这句话等同于Class.forName("class").newInstance()通过反射生成一个对象
         Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
         interceptorInstance.setProperties(properties);
         //List<Interceptor> interceptors = new ArrayList<>(); 集合插入对象
         configuration.addInterceptor(interceptorInstance);
       }
     }
   

插件调用流程

1. 获取session

   SqlSession sqlSession1 = sqlSessionFactory.openSession();
   

2. 新增执行器

   private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
       //新建一个执行器
       final Executor executor = configuration.newExecutor(tx, execType);
       return new DefaultSqlSession(configuration, executor, autoCommit);
   }
   

3. 初始化执行器

   public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
     ....
     //通过循环代理 一层层的包装来实现调用链
     executor = (Executor) interceptorChain.pluginAll(executor);
     return executor;
   }
   

4. 动态生成拦截器代理

   public Object pluginAll(Object target) {
       //先进后出
       for (Interceptor interceptor : interceptors) {
        //动态生成代理类等于Proxy.newProxyInstance 像洋葱一样 代理了另一个代理类
         target = interceptor.plugin(target);
       }
       return target;
     }
   

   public interface Interceptor {
     Object intercept(Invocation invocation) throws Throwable;
     //default 关键字方法的默认实现 实现类无需实现这个方法  
     default Object plugin(Object target) {
       //静态工厂方法获取一个代理类
       return Plugin.wrap(target, this);
     }
   }
   

   Plugin implements InvocationHandler

   //静态工厂方法 生成代理类
   public static Object wrap(Object target, Interceptor interceptor) {
     //通过解析拦截器Intercepts注解与Signature注解
     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;
   }
   

   @Override
   public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
     try {
       //判断此拦截器需要拦截的方法是否与需要执行的方法相同 
       Set<Method> methods = signatureMap.get(method.getDeclaringClass());
       if (methods != null && methods.contains(method)) {
         //执行拦截器逻辑
         return interceptor.intercept(new Invocation(target, method, args));
       }
       return method.invoke(target, args);
     } catch (Exception e) {
       throw ExceptionUtil.unwrapThrowable(e);
     }
   }
   

插件调用链视图

image-20200702165404163.png

精简代码核心流程

看一下删除了各种设计模式之后的代码 只是展示思路用

public void testInterceptor() {
    Object target = 12;
    List<Interceptor> interceptorList = new ArrayList<>();
    interceptorList.add(new InterceptorOne());
    interceptorList.add(new InterceptorTwo());
    for(Interceptor interceptor : interceptorList){
        //管理类
        PluginInvocation pluginInvocation = new PluginInvocation(target, interceptor);
        //代理类
        target =  Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), pluginInvocation);
    }
    //模拟触发代理
    System.out.println(target.equals(12));
}

InterceptorOne

public class InterceptorOne implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        System.out.println("执行拦截器 1");
        return null;
    }
}

InterceptorTwo

public class InterceptorTwo implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        System.out.println("执行拦截器2");
        return null;
    }
}

PluginInvocation

public class PluginInvocation implements InvocationHandler {

    //被代理对象
    private Object targetObject;
    private Interceptor interceptor;

    public PluginInvocation(Object o, Interceptor interceptor) {
        this.targetObject = o;
        this.interceptor = interceptor;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        //执行拦截器逻辑 这里会有判断是否需要拦截的逻辑
        interceptor.intercept(null);
        //执行目标对象的方法
        return method.invoke(targetObject, args);
    }

}

pageHelper插件怎么分页的

什么对象可以被拦截？官网如下

MyBatis 允许你在映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：

• Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)

  (对应插件调用流程-3.初始化执行器 的时候会用代理模式包装Executor对象)

• ParameterHandler (getParameterObject, setParameters)

• ResultSetHandler (handleResultSets, handleOutputParameters)

• StatementHandler (prepare, parameterize, batch, update, query)

应用场景

水平分表 权限控制 数据的加解密 分页插件