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Preface

As an excellent ORM framework with a wide range of applications, Mybatis has become an almost standard part of the JavaWeb world. This framework has strong flexibility and provides an easy-to-use plug-in extension mechanism in the four major components (Executor, StatementHandler, ParameterHandler, ResultSetHandler). Mybatis operates on the persistence layer with the help of four core objects. MyBatis supports the use of plug-ins to intercept the four core objects. For mybatis, plug-ins are interceptors to enhance the function of core objects, which is essentially realized with the help of underlying dynamic proxies. In other words, the four major objects in MyBatis are all proxy objects.

A brief introduction to the four Core objects

Four core objects of MyBatis

ParameterHandler: a parameter object that handles SQL

ResultSetHandler: processing the returned result set of SQL

StatementHandler: the processing object of the database, used to execute SQL statements

The executor of Executor:MyBatis, which is used to perform add, delete, modify and search operations.

Principle of Mybatis plug-in

The plug-in of Mybatis handles the interception with the help of the mode of chain of responsibility, wraps the target object with dynamic proxies to achieve the purpose of interception acting on the scope object of Mybatis.

intercept

How exactly does the plug-in intercept and attach additional functionality?

In the case of ParameterHandler

Public ParameterHandler newParameterHandler (MappedStatement mappedStatement, Object object, BoundSql sql, InterceptorChain interceptorChain) {ParameterHandler parameterHandler = mappedStatement.getLang (). CreateParameterHandler (mappedStatement,object,sql); parameterHandler = (ParameterHandler) interceptorChain.pluginAll (parameterHandler); return parameterHandler;} public Object pluginAll (Object target) {for (Interceptor interceptor: interceptors) {target = interceptor.plugin (target);} return target;}

InterceptorChain holds all the interceptors, which is created when mybatis is initialized. Call the interceptor in the interceptor chain to intercept or enhance the target in turn. The target in interceptor.plugin (target) can be understood as the four major objects in mybatis. The returned target is the object that has been heavily proxied.

Plug-in interface

Mybatis plug-in Interface-Interceptor

1.Intercept method, the core method of the plug-in

2.plugin method to generate a proxy object for target

3.setProperties method, passing the parameters required by the plug-in

Plug-in instance

Plug-in development requires the following steps

Custom plug-ins need to implement the above interfaces and add @ Intercepts annotations (which core components are declared plug-ins and which methods are extended) to configure the plug-ins in the xml file

/ * * plug-in signature, telling the mybatis single money plug-in which method to intercept that object * * / @ Intercepts ({@ Signature (type = ResultSetHandler.class,method = "handleResultSets", args = Statement.class)}) public class MyFirstInterceptor implements Interceptor {/ * * @ Description intercepts the target object's target method * * / @ Override public Object intercept (Invocation invocation) throws Throwable {System.out.println ("intercepted target object:" + invocation.getTarget ()) Object object = invocation.proceed (); return object;} / * * @ Description wraps the target object to create a proxy object for the target object * @ Param target is the object to be intercepted * @ Return proxy object * / @ Override public Object plugin (Object target) {System.out.println ("target object to be wrapped:" + target); return Plugin.wrap (target,this) } / * * get the properties of the configuration file * * / @ Override public void setProperties (Properties properties) {System.out.println ("initialization parameters of plug-in configuration:" + properties);}}

Configure plug-ins in mybatis.xml

Call the query method and the query method returns ResultSet

Public class MyBatisTest {public static SqlSessionFactory sqlSessionFactory = null; public static SqlSessionFactory getSqlSessionFactory () {if (sqlSessionFactory = = null) {String resource = "mybatis-config.xml"; try {Reader reader = Resources.getResourceAsReader (resource); sqlSessionFactory = new SqlSessionFactoryBuilder (). Build (reader);} catch (IOException e) {e.printStackTrace ();}} return sqlSessionFactory;} public void testGetById () {SqlSession sqlSession = this.getSqlSessionFactory (). OpenSession (); PersonMapper personMapper = sqlSession.getMapper (PersonMapper.class); Person person=personMapper.getById (2001) System.out.println (person.toString ());} public static void main (String [] args) {new MyBatisTest () .testGetById ();}}

Output result

Initialization parameters for plug-in configuration: {name=Bob} target object to be wrapped by org.apache.ibatis.executor.CachingExecutor@754ba872: target object to be wrapped by org.apache.ibatis.scripting.defaults.DefaultParameterHandler@192b07fd: target object to be wrapped by org.apache.ibatis.executor.resultset.DefaultResultSetHandler@7e0b0338: target object to be intercepted by org.apache.ibatis.executor.statement.RoutingStatementHandler@1e127982: org.apache.ibatis.executor.resultset.DefaultResultSetHandler@7e0b0338Person {id=2001 Username='Tom', email='email@0', gender='F'}

Multi-plug-in development process

1. When creating a proxy object, wrap it in the order in which the plug-in is configured

two。 After the target method is executed, it is executed in accordance with the reverse of the agent

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