mybatis-plugin的实现原理(源码分析)

Posted 2021-12-18 八阿哥克星

mybatis的plugin插件就是拦截器，可以对ParameterHandler、ResultSetHandler、StatementHandler、Executor共4个接口对象内的方法进行拦截，因此，自定义插件，必须要实现org.apache.ibatis.plugin.Interceptor接口。重写方法interceptor()

package org.apache.ibatis.plugin;

import java.util.Properties;

public interface Interceptor 
    Object intercept(Invocation var1) throws Throwable;

    default Object plugin(Object target) 
        return Plugin.wrap(target, this);
    

    default void setProperties(Properties properties) 
    

上图中plugin()方法，是给目标对象生成一个动态代理对象，再有代理对象执行拦截方法，调用重写的interceptor()。

@Intercepts(@Signature(
    type = StatementHandler.class,
    method = "prepare",
    args = Connection.class, Integer.class
))

另外，注解@Interceptors是必加的，目的就是明确该拦截器的要拦截的对象甚至方法，type是要拦截的类，method是方法名称，args是参数。

Interceptor拦截器注册时机是在ParameterHandler、ResultSetHandler、StatementHandler、Executor共4个接口对象创建的时候，configuration对象中：

 public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) 
        ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
        //将目标对象加到拦截器链中
        parameterHandler = (ParameterHandler)this.interceptorChain.pluginAll(parameterHandler);
        return parameterHandler;
    

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

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

    public Executor newExecutor(Transaction transaction) 
        return this.newExecutor(transaction, this.defaultExecutorType);
    

interceptorChain.pluginAll()方法：该方法在创建上述4个接口对象时调用，其含义为给这些接口对象注册拦截器功能，注意是注册，而不是执行拦截。

 public Object pluginAll(Object target) 
        Interceptor interceptor;
        //for循环代表了只要是插件，都会以责任链的方式逐一执行
        for(Iterator var2 = this.interceptors.iterator(); var2.hasNext(); target = interceptor.plugin(target)) 
            interceptor = (Interceptor)var2.next();
        

        return target;
    

　　拦截器执行时机：plugin()方法注册拦截器后，那么，在执行上述4个接口对象内的具体方法时，就会自动触发拦截器的执行，也就是插件的执行。