拦截器的实现都是基于代理的设计模式实现的,简单的说就是要创造一个目标类的代理类,在代理类中执行目标类的方法并在方法之前执行拦截器代码。 首先,先不管mybatis的源码是怎么设计的,先假设一下自己要做一个拦截器应该怎么做。下面我们就利用JDK的动态代理自己设计一个简单的拦截器。
将被拦截的目标接口:
public interface Target {
public void execute();
}
目标接口的一个实现类:
public class TargetImpl implements Target {
public void execute() {
System.out.println("Execute");
}
}
利用JDK的动态代理实现拦截器:
public class TargetProxy implements InvocationHandler {
private Object target;
private TargetProxy(Object target) {
this.target = target;
}
//生成一个目标对象的代理对象
public static Object bind(Object target) {
return Proxy.newProxyInstance(target.getClass() .getClassLoader(),
target.getClass().getInterfaces(),
new TargetProxy(target));
}
//在执行目标对象方法前加上自己的拦截逻辑
@Override
public Object invoke(Object proxy, Method method,
Object[] args) throws Throwable {
System.out.println("Begin");
return method.invoke(target, args);
}
}
客户端调用:
public class Client {
public static void main(String[] args) {
//没有被拦截之前
Target target = new TargetImpl();
target.execute(); //Execute
//拦截后
// 当代理对象调用真实对象的方法时,其会自动的跳转到代理对象关联的handler对象的invoke方法来进行调用
target = (Target)TargetProxy.bind(target);
target.execute();
//Begin
//Execute
}
上面的设计有几个非常明显的不足。首先,拦截逻辑被写死在代理对象中:
public Object invoke(Object proxy, Method method,
Object[] args) throws Throwable {
//拦截逻辑被写死在代理对象中,导致客户端无法灵活的设置自己的拦截逻辑
System.out.println("Begin");
return method.invoke(target, args);
}
我们可以将拦截逻辑封装到一个类中,客户端在调用TargetProxy的bind()方法的时候将拦截逻辑一起当成参数传入:
定义一个拦截逻辑封装的接口Interceptor,这才是真正的拦截器接口。
public interface Interceptor {
public void intercept();
}
那么我们的代理类就可以改成:
public class TargetProxy implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private TargetProxy(Object target, Interceptor interceptor) {
this.target = target;
this.interceptor = interceptor;
}
//将拦截逻辑封装到拦截器中,有客户端生成目标类的代理类的时候一起传入,这样客户端就可以设置不同的拦截逻辑。
public static Object bind(Object target, Interceptor interceptor) {
return Proxy.newProxyInstance(target.getClass().getClassLoader(),
target.getClass().getInterfaces(),
new TargetProxy(target, interceptor));
}
public Object invoke(Object proxy, Method method,
Object[] args) throws Throwable {
//执行客户端定义的拦截逻辑
interceptor.intercept();
return method.invoke(target, args);
}
客户端调用代码:
/客户端可以定义各种拦截逻辑
Interceptor interceptor = new Interceptor() {
public void intercept() {
System.out.println("Go Go Go!!!");
}
};
target = (Target)TargetProxy.bind(target, interceptor);
target.execute();
当然,很多时候我们的拦截器中需要判断当前方法需不需要拦截,或者获取当前被拦截的方法参数等。我们可以将被拦截的目标方法对象,参数信息传给拦截器。
拦截器接口改成:
public interface Interceptor {
public void intercept(Method method, Object[] args);
}
在代理类执行的时候可以将当前方法和参数传给拦截,即TargetProxy的invoke方法改为:
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
interceptor.intercept(method, args);
return method.invoke(target, args);
}
在Java设计原则中有一个叫做迪米特法则,大概的意思就是一个类对其他类知道得越少越好。其实就是减少类与类之间的耦合强度。这是从类成员的角度去思考的。 什么叫越少越好,什么是最少?最少就是不知道。 所以我们是不是可以这么理解,一个类所要了解的类应该越少越好呢?
对于上面的例子,Interceptor接口中需要使用intercept方法传过去Method类,那么也需要了解它。那么既然Interceptor都需要使用Method,还不如将Method的执行也放到Interceptor中,不再让TargetProxy类对其了解。Method的执行需要target对象,所以也需要将target对象给Interceptor。将Method,target和args封装到一个对象Invocation中,将Invocation传给Interceptor。
public interface Interceptor {
Object intercept(Invocation invocation)throws Throwable;
}
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;
}
//将自己成员变量的操作尽量放到自己内部,不需要Interceptor获得自己的成员变量再去操作它们,
//除非这样的操作需要Interceptor的其他支持。然而这里不需要。
public Object proceed() throws InvocationTargetException, IllegalAccessException {
return method.invoke(target, args);
}
public Object getTarget() {
return target;
}
public void setTarget(Object target) {
this.target = target;
}
public Method getMethod() {
return method;
}
public void setMethod(Method method) {
this.method = method;
}
public Object[] getArgs() {
return args;
}
public void setArgs(Object[] args) {
this.args = args;
}
}
public interface Target {
public void execute();
}
public class TargetProxy implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private TargetProxy(Object target, Interceptor interceptor) {
this.target = target;
this.interceptor = interceptor;
}
//将拦截逻辑封装到拦截器中,有客户端生成目标类的代理类的时候一起传入,这样客户端就可以设置不同的拦截逻辑。
public static Object bind(Object target, Interceptor interceptor) {
return Proxy.newProxyInstance(target.getClass().getClassLoader(),
target.getClass().getInterfaces(),
new TargetProxy(target, interceptor));
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
return interceptor.intercept(new Invocation(target, method, args));
}
}
public class Client {
public static void main(String[] args) {
/**
* 相当于:
Interceptor interceptor2 = new Interceptor() {
@Override
public Object intercept(Invocation invocation) throws Throwable {
System.out.println("Go Go Go!!!");
return invocation.proceed();
}
} ;
*/
Interceptor interceptor = invocation -> {
System.out.println("Go Go Go!!!");
return invocation.proceed();
};
Target target = (Target) TargetProxy.bind(new TargetImpl(), interceptor);
target.execute();
}
}
根据迪米特法则来讲,其实客户端根本不需要了解TargetProxy类。将绑定逻辑放到拦截器内部,客户端只需要和拦截器打交道就可以了。
拦截器接口变为:
public interface Inteceptor {
Object intercept(Invocation invocation)throws Throwable;
Object register(Object target);
}
拦截器实现类:
public class InteceptorImpl implements Inteceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
System.out.println("Go Go Go!!!");
return invocation.proceed();
}
@Override
public Object register(Object target) {
return TargetProxy.bind(target, this);
}
}
测试类:
public class Client {
public static void main(String[] args) {
Inteceptor interceptor = new InteceptorImpl();
Target target = (Target) interceptor.register(new TargetImpl());
target.execute();
}
}
好了,通过一系列调整,设计已经挺好了,不过上面的拦截器还是有一个很大的不足, 那就是拦截器会拦截目标对象的所有方法,然而这往往是不需要的,我们经常需要拦截器拦截目标对象的指定方法。 我们利用在Interceptor上加注解解决。
假设目标对象接口有多个方法:
public interface Target {
public void execute1();
public void execute2();
}
public class TargetImpl implements Target {
@Override
public void execute1() {
System.out.println("Execute");
}
@Override
public void execute2() {
System.out.println("Execute");
}
}
首先简单的定义一个注解:
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MethodName {
public String value();
}
在拦截器的实现类加上该注解:
public interface Interceptor {
Object intercept(Invocation invocation)throws Throwable;
Object register(Object target);
}
@MethodName("execute1")
public class InteceptorImpl implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
System.out.println("Go Go Go!!!");
return invocation.proceed();
}
@Override
public Object register(Object target) {
return TargetProxy.bind(target, this);
}
}
在TargetProxy中判断interceptor的注解,看是否实行拦截:
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class TargetProxy implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private TargetProxy(Object target, Interceptor interceptor) {
this.target = target;
this.interceptor = interceptor;
}
//将拦截逻辑封装到拦截器中,有客户端生成目标类的代理类的时候一起传入,这样客户端就可以设置不同的拦截逻辑。
public static Object bind(Object target, Interceptor interceptor) {
return Proxy.newProxyInstance(target.getClass().getClassLoader(),
target.getClass().getInterfaces(),
new TargetProxy(target, interceptor));
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodName methodName = this.interceptor.getClass().getAnnotation(MethodName.class);
if (methodName == null) {
throw new NullPointerException("xxxx");
}
//如果注解上的方法名和该方法名一样,才拦截
String name = methodName.value();
if (name.equals(method.getName())){
//拦截方法后 加上自己的逻辑
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(this.target, args);
}
}
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
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;
}
//将自己成员变量的操作尽量放到自己内部,不需要Interceptor获得自己的成员变量再去操作它们,
//除非这样的操作需要Interceptor的其他支持。然而这里不需要。
public Object proceed() throws InvocationTargetException, IllegalAccessException {
return method.invoke(target, args);
}
public Object getTarget() {
return target;
}
public void setTarget(Object target) {
this.target = target;
}
public Method getMethod() {
return method;
}
public void setMethod(Method method) {
this.method = method;
}
public Object[] getArgs() {
return args;
}
public void setArgs(Object[] args) {
this.args = args;
}
}
测试类:
public class Client {
public static void main(String[] args) {
Interceptor interceptor = new InteceptorImpl();
Target target = (Target) interceptor.register(new TargetImpl());
target.execute1();
target.execute2();
}
}
打印结果,只拦截了execute1()方法:
Go Go Go!!!
Execute1
Execute2
Process finished with exit code 0
OK,上面的一系列过程其实都是mybatis的拦截器代码结构,上面的TargetProxy其实就是mybatis的Plugin类。Interceptor和Invocation几乎一样。只是mybatis的Interceptor支持的注解更加复杂。
MyBatis的Plugin类,看到里面的两个方法,现在就不那么陌生了。
public class Plugin implements InvocationHandler {
private Object target;
private Interceptor interceptor;
private Map<Class<?>, Set<Method>> 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) {
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());
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
...
}
mybatis最终是通过将自定义的Interceptor配置到xml文件中:
<!-- 自定义处理Map返回结果的拦截器 -->
<plugins>
<plugin interceptor="com.chm.inteceptor.PageInterceptor">
</plugins>