我们首先介绍一下Mybatis插件相关的几个类，并对源码进行了简单的分析。
Mybatis插件相关的接口或类有：Intercept、InterceptChain、Plugin和Invocation，这几个接口或类实现了整个Mybatis插件流程。
Interceptor：一个接口，是实现自己功能需要实现的接口
源码如下：
/**
* @author Clinton Begin
*/
public interface Interceptor {
//在此方法中实现自己需要的功能，最后执行invocation.proceed()方法，实际就是调用method.invoke(target, args)方法，调用代理类
Object intercept(Invocation invocation) throws Throwable;
//这个方法是将target生成代理类
Object plugin(Object target);
//在xml中注册Intercept是配置一些属性
void setProperties(Properties properties);
}
InterceptorChain：有一个List<Interceptor> interceptors变量，来保存所有Interceptor的实现类
源码如下：
/**
* @author Clinton Begin
*/
public class InterceptorChain {
//插件拦截器链
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
//把target变成代理类，这样在运行target方法之前需要运行Plugin的invoke方法
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
Invocation：一个比较简单的类，主要的功能就是根据构造函数类执行代理类
源码如下：
/**
* @author Clinton Begin
*/
public class Invocation {
private Object target;
private Method method;
private Object[] args;
public Invocation(Object target, Method method, Object[] args) {
this.target = target;
this.method = method;
this.args = args;
}
public Object getTarget() {
return target;
}
public Method getMethod() {
return method;
}
public Object[] getArgs() {
return args;
}
//其实mybatis的Interceptor最终还是调用的method.invoke方法
public Object proceed() throws InvocationTargetException, IllegalAccessException {
return method.invoke(target, args);
}
}
Plugin：Mybatis插件的核心类，它实现了代理接口InvocationHandler，是一个代理类
源码详解如下：
/**
* @author Clinton Begin
*/
//这个类是Mybatis拦截器的核心,大家可以看到该类继承了InvocationHandler
//又是JDK动态代理机制
public class Plugin implements InvocationHandler {
//目标对象
private Object target;
//拦截器
private Interceptor interceptor;
//记录需要被拦截的类与方法
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;
}
//一个静态方法,对一个目标对象进行包装，生成代理类。
public static Object wrap(Object target, Interceptor interceptor) {
//首先根据interceptor上面定义的注解 获取需要拦截的信息
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
//如果长度为>0 则返回代理类 否则不做处理
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;
}
//在执行Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类的方法时会调用这个方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//通过method参数定义的类 去signatureMap当中查询需要拦截的方法集合
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
//判断是否是需要拦截的方法，如果需要拦截的话就执行实现的Interceptor的intercept方法，执行完之后还是会执行method.invoke方法，不过是放到interceptor实现类中去实现了
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
//不拦截 直接通过目标对象调用方法
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//根据拦截器接口（Interceptor）实现类上面的注解获取相关信息
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
//获取注解信息
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
// issue #251
//为空则抛出异常
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
//获得Signature注解信息
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
//循环注解信息
for (Signature sig : sigs) {
//根据Signature注解定义的type信息去signatureMap当中查询需要拦截方法的集合
Set<Method> methods = signatureMap.get(sig.type());
//第一次肯定为null 就创建一个并放入signatureMap
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
//找到sig.type当中定义的方法 并加入到集合
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;
}
//根据对象类型与signatureMap获取接口信息
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
//循环type类型的接口信息 如果该类型存在与signatureMap当中则加入到set当中去
while (type != null) {
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
type = type.getSuperclass();
}
//转换为数组返回
return interfaces.toArray(new Class<?>[interfaces.size()]);
}
}
对Mybatis插件有一定了解的人应该知道，插件拦截的类是Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类，为什么会这样呢？在Configuration类中看一下代码就明白了，可以看到在初始化Executor、ParameterHandler、ResultSetHandler和StatementHandler都会调用interceptorChain.pluginAll（）这个函数，其实这样之后各个接口的实现类就被代理类生成为目标类了。
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;
}