插件
- 使用
- 确定需要拦截的签名

插件

使用

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

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<Interceptor> interceptors = new ArrayList<Interceptor>();

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

  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

}

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<Class<?>, Set<Method>> signatureMap;

  private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
    this.target = target;
    this.interceptor = interceptor;
    this.signatureMap = signatureMap;
  }

  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;
  }

  public 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);
    }
  }

  private static Map<Class<?>, Set<Method>> 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<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 {
        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<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<Class<?>>();
    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

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

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?)

    <plugins>
        <plugin interceptor="com.jannal.mybatis.plugin.MyPlugin">
            <property name="jannal" value="jannalpro" />
        </plugin>
    </plugins>

     /**
 * @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.....

mybatis之插件机制

插件

使用

确定需要拦截的签名

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