实现原理
首先Plugin必须实现Interceptor
常拦截以下类或接口中的方法:
Executor
ParameterHandler
ResultSetHandler
-
StatementHandler
与Plugin有关的类:plugin
、InterceptorChain
@Intercepts({@Signature(type = Executor.class,
method ="query",
args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
public class CustomPlugins implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
//最终plugin插件调用的是这个方法
MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
BoundSql boundSql = mappedStatement.getBoundSql(invocation.getArgs()[1]);
System.out.println(String.format("plugin output sql = %s , param=%s", boundSql.getSql(),boundSql.getParameterObject()));
//放行 该方法
return invocation.proceed();
}
@Override
public Object plugin(Object o) {
Object obj= Plugin.wrap(o,this);
return obj;
}
@Override
public void setProperties(Properties properties) {
//常用于将配置中的参数赋值给类的实例变量
String value = (String) properties.get("name");
System.out.println(value);
}
}
//InterceptorChain
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
//这里的intecetor是Plugin 也即是自定义插件中plugin方法
//这里的target是Executor、ParameterHandler、ResultSetHandler、StatementHandler接口的实现类 具体是什么 要看@Interceptors中拦截的接口
//这里的target是通过for循环不断赋值的,也就是说如果有多个拦截器,那么如果我用P表示代理,生成第 //一次代理为P(target),生成第二次代理为P(P(target)),生成第三次代理为P(P(P(target))),不断 //嵌套下去,这就得到一个重要的结论:<plugins>...</plugins>中后定义的<plugin>实际其拦截器方法 //先被执行,因为根据这段代码来看,后定义的<plugin>代理实际后生成,包装了先生成的代理,自然其代理方 //法也先执行.也即是interceptor的执行顺序 后定义先执行
//这里的plugin方法实际上是调用对应拦截器类的重载方法
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
//添加拦截器至集合
interceptors.add(interceptor);
}
// 列举所有的拦截器类
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
//Plugin
public static Object wrap(Object target, Interceptor interceptor) {
//获取拦截器的签名信息,具体代码如下
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
//如果数组长度大于0 也就是表示type有父接口 可以根据其接口生成代理对象
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
//否则 返回target对象本身
return target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//method#getDeclaringClass() 获取method对应的class类
//以CustomPlugin为例,此时method为query 得到的的declaringClass为Executor
//此时签名Map中可以得到key为Executor的value
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
//这里调用拦截器中的intercept方法 拦截器对应具体的实现 如此时是CustomPlugin
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//getSignatureMap方法
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
// issue #251
//如果拦截器类没有Intercepts注解则抛出异常
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
//以CustomPlugins为例@Intercepts({@Signature(type = Executor.class,
//method ="query",
//args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
//type=Executor.class,method=query args=...
Signature[] sigs = interceptsAnnotation.value();
//如果存在以type为key的map 则获取其拦截的方法 否则执行map的put方法 为key赋值
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
//利用反射 获取Executor中方法为query,参数为sig.args的方法
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);
}
}
//返回签名map
return signatureMap;
}
//getInterfaces
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
while (type != null) {
//如此时type为Executor的具体实例 CachingExecutor
//type.getInterfaces 得到的是Executor
//刚好Executor.class 为map的key
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
//向上查询 查询其父类
type = type.getSuperclass();
}
return interfaces.toArray(new Class<?>[interfaces.size()]);
}
使用方式
自定义插件常通过实现Interceptor接口的方式实现
//表示拦截的是哪个类的含有几个参数的方法
@Intercepts({@Signature(type = Executor.class,
method ="query",
args={MappedStatement.class,Object.class,RowBounds.class,ResultHandler.class})})
public class CustomPlugins implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
//一系列的操作 然后拦截器放行
return invocation.proceed();
}
@Override
public Object plugin(Object o) {
//o表示拦截接口的子类 this表示CustomPlugins
return Plugin.wrap(o,this);
}
@Override
public void setProperties(Properties properties) {
//读取配置插件时 给出的参数信息
//常用于将配置中的参数赋值给类的实例变量
String value = (String) properties.get("name");
System.out.println(value);
}
}
配置plugin的两种形式:
- 基于xml的形式
//mybatis.xml
<plugins>
<plugin interceptor="com.wojiushiwo.dal.plugins.CustomPlugins">
<property name="name" value="wojiushiwo"/>
</plugin>
</plugins>
- 基于Java config的形式
//使用的时候直接在注入即可
@Bean
public SqlSessionFactory localSessionFactoryBean() throws Exception {
SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
sqlSessionFactoryBean.setDataSource(dataSource);
//将自定义的typeHandler设置到mybatis配置文件中
sqlSessionFactoryBean.setTypeHandlers(new TypeHandler[]{new CustomTypeHandler()});
//将自定义的插件设置到mybatis配置中
sqlSessionFactoryBean.setPlugins(new Interceptor[]{plugin()});
SqlSessionFactory factory = sqlSessionFactoryBean.getObject();
factory.getConfiguration().setLazyLoadingEnabled(true);
factory.getConfiguration().setAggressiveLazyLoading(false);
factory.getConfiguration().setProxyFactory(new CglibProxyFactory());
return factory;
}
private Interceptor plugin(){
CustomPlugins customPlugins=new CustomPlugins();
Properties properties=new Properties();
//这里可以设置自定义参数
properties.setProperty("name","wojiushiwo");
customPlugins.setProperties(properties);
return customPlugins;
}
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