mybatis的plugin

1.Mybatis-Plugin的设计思路

听起来一个挺神奇的单词,插件。说白了就是使用了Jdk自带的动态代理.在需要的时候进行代理。AOP怎么用，他就怎么用。

Plugin类等价于InvocationHandler.Mybatis在调用Jdk代码如下:

Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));

Plugin类继承InvocationHandler接口

public class Plugin implements InvocationHandler {

  private final Object target;
  private final Interceptor interceptor;
  private final 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;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

invoke方法执行的时候,也就是intercept被调用的时候.

用一句成语来讲，借鸡下蛋。借尸还魂(这个词还有点吓人)

2.写一个例子.使用Intecept接口,但不应用在Mybatis框架内。

思路:① 写一个接口+一个类②写一个Inteceptor对该接口进行增强。

--------------------接口----------------------------------

package org.mybatis.example.plugin;

public interface HelloWorld {

    void sayHello(Integer v);
}

-------------------实现类---------------------------------

package org.mybatis.example.plugin;

public class HelloWorldImpl implements HelloWorld {

    public void sayHello(Integer v){
        System.out.println("Hello World!");
    }

    public static void main(String[] args) {

        PluginUsedWithoutMyBatis plugin = new PluginUsedWithoutMyBatis();

        HelloWorld helloWorld = (HelloWorld) plugin.plugin(new HelloWorldImpl());

        helloWorld.sayHello(1);
    }
}

--------------------Interceptor-----------------------------------

@Intercepts(@Signature(type= HelloWorld.class,method="sayHello",args={Integer.class}))
public class PluginUsedWithoutMyBatis implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {

        String methodName =invocation.getMethod().getName();

        Object result =null;
        if(methodName.equals("sayHello")){

            System.out.println("method Begin:");

            result =invocation.proceed();
            System.out.println("method Begin:");
        }else{
            result =invocation.proceed();
        }

        return result;
    }

    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target,this);
    }

    @Override
    public void setProperties(Properties properties) {

    }

    public static void main(String[] args) {

    }
}

执行HelloWorldImpl中的Main方法，得到一段朴素的代码增强结果。

method Begin:
Hello World!
method Begin:

PluginUsedWithoutMyBatis类中需要注意的：

@Intercepts、@Signature这两个注解，限定了可以增强的接口，以及方法名称以及参数。
理解成过滤器。在Mybatis的Plugins配置中，所有的Plugin都配置在一起。根据Signature决定Interceptor对谁增强。

问题:

如果两个Interceptor都对同一个对象增强,结果如何?

写了另外一个代理类，对HelloWorldImpl进行二次增强，结果如下:

enhance Second Begin:
enhance First Begin:
Hello World!
enhance First end:
enhance Second end:

可以看到两次增强的结果。

3、回到Mybatis,是否可以对所有的MyBatis接口增强？

Mybatis的Configruation类，只在几个对象的创建时做了增强

StatementHandler 、ResultSetHandler 、ParameterHandler 、Executor 

  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);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

    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);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : 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);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

4、现有增强插件的实现:

Executor的query、update时候可以监控sql执行时间。监控慢SQL

Executor的query、update catch异常，当发生异常时，可以打印sql以及执行参数。以及具体错误消息。