首先Plugin必須實現Interceptor
常攔截以下類或接口中的方法:
Executor
ParameterHandler
ResultSetHandler
StatementHandler
與Plugin有關的類:plugin
、InterceptorChain
//自定義一個插件類
@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 {
//最終plugin插件調用的是這個方法
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 該方法
return invocation.proceed();
}
@Override
public Object plugin(Object o) {
Object obj= Plugin.wrap(o,this);
return obj;
}
@Override
public void setProperties(Properties properties) {
//常用於將配置中的參數賦值給類的實例變量
String value = (String) properties.get("name");
System.out.println(value);
}
}
//InterceptorChain類
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
//這裏的intecetor是Plugin 也即是自定義插件中plugin方法
//這裏的target是Executor、ParameterHandler、ResultSetHandler、StatementHandler接口的實現類 具體是什麼 要看@Interceptors中攔截的接口
//這裏的target是通過for循環不斷賦值的,也就是說如果有多個攔截器,那麼如果我用P表示代理,生成第 //一次代理爲P(target),生成第二次代理爲P(P(target)),生成第三次代理爲P(P(P(target))),不斷 //嵌套下去,這就得到一個重要的結論:<plugins>...</plugins>中後定義的<plugin>實際其攔截器方法 //先被執行,因爲根據這段代碼來看,後定義的<plugin>代理實際後生成,包裝了先生成的代理,自然其代理方 //法也先執行.也即是interceptor的執行順序 後定義先執行
//這裏的plugin方法實際上是調用對應攔截器類的重載方法
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
//添加攔截器至集合
interceptors.add(interceptor);
}
// 列舉所有的攔截器類
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
//Plugin類
public static Object wrap(Object target, Interceptor interceptor) {
//獲取攔截器的簽名信息,具體代碼如下
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
//如果數組長度大於0 也就是表示type有父接口 可以根據其接口生成代理對象
if (interfaces.length > 0) {
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
//否則 返回target對象本身
return target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//method#getDeclaringClass() 獲取method對應的class類
//以CustomPlugin爲例,此時method爲query 得到的的declaringClass爲Executor
//此時簽名Map中可以得到key爲Executor的value
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
//這裏調用攔截器中的intercept方法 攔截器對應具體的實現 如此時是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
//如果攔截器類沒有Intercepts註解則拋出異常
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
//以CustomPlugins爲例@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爲key的map 則獲取其攔截的方法 否則執行map的put方法 爲key賦值
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 獲取Executor中方法爲query,參數爲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);
}
}
//返回簽名map
return signatureMap;
}
//getInterfaces
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
while (type != null) {
//如此時type爲Executor的具體實例 CachingExecutor
//type.getInterfaces 得到的是Executor
//剛好Executor.class 爲map的key
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
//向上查詢 查詢其父類
type = type.getSuperclass();
}
return interfaces.toArray(new Class<?>[interfaces.size()]);
}