本来想直接写Mybatis插件PageHelper的，但里面涉及了mybatis的plugin，所以就顺便了解下Mybatis的plugin。通常我们使用 Mybatis插件时，都会使用下面的配置。配置如下：
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <plugins> <plugin interceptor="org.mybatis.example.ExamplePlugin"> <property name="someProperty" value="100"/> </plugin> </plugins> </configuration>Mybatis的插件配置会存储在Configuration内部，初始化时，会读取这些插件，保存于Configuration对象的InterceptorChain中。Configuration的InterceptorChain定义如下：
public class Configuration { protected final InterceptorChain interceptorChain = new InterceptorChain(); }2.如何编写一个插件
首先需要实现org.apache.ibatis.plugin.Interceptor接口。Interceptor接口定义如下：
下面编写一个插件，代码如下：
@Intercepts({@Signature( type= Executor.class, method = "update", args = {MappedStatement.class,Object.class})}) //要拦截Executor接口内的update()方法，参数类型为args列表 public class ExamplePlugin implements Interceptor { public Object intercept(Invocation invocation) throws Throwable { return invocation.proceed(); } // 用于给target创建一个JDK的动态代理对象，触发intercept()方法 public Object plugin(Object target) { return Plugin.wrap(target, this); } public void setProperties(Properties properties) { } }通过上面发现Plugin这个类很重要，接下来在看看它的源码，代码如下：
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) { // 创建JDK动态代理对象 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 { Set<Method> methods = signatureMap.get(method.getDeclaringClass()); //判断执行方法是否需要拦截 if (methods != null && methods.contains(method)) { //调用Interceptor的intercept（）方法 return interceptor.intercept(new Invocation(target, method, args)); } return method.invoke(target, args); } catch (Exception e) { throw ExceptionUtil.unwrapThrowable(e); } } //....... }3.Mybatis可以拦截哪些接口？
要知道这个问题 ，看看Configuration的源码就行了，代码如下：
public class Configuration { //... 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 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 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 Executor newExecutor(Transaction transaction) { return newExecutor(transaction, defaultExecutorType); } 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; } //... }从上面可以看出，Mybatis只拦截ParameterHandler、ResultSetHandler、StatementHandler、Executor共4个接口对象内的方法。
