问题的提出：自定义一个注解，如@MyLog，当把此注解加在函数上时，该函数的调用会被自动日志。
解题思路：创建函数所在对象的动态代理，当该函数被调用时，在代理中进行日志。
两种方法：方法一使用JDK动态代理，只能对实现了接口的类进行代理；方法二使用CGLiB动态代理，可以对普通类进行代理。
1. 自定义MyLog注解
1 package org.example.annotation;
2
3 import java.lang.annotation.*;
4
5 @Target(ElementType.METHOD)
6 @Retention(RetentionPolicy.RUNTIME)
7 @Documented
8 public @interface MyLog {
9 String beginMsg() default "执行开始...";
10 String endMsg() default "执行结束！！！";
11 }
方法一：使用JDK动态代理。
先定义一个接口：
1 package org.example.business;
2
3 public interface IPerson {
4 boolean eat(String food);
5
6 String walk(String start, String finish);
7
8 void sleep(int seconds);
9 }
再实现上述接口，注意实现类的工厂方法createPerson():
1 package org.example.business;
2
3 import org.example.annotation.MyLog;
4 import org.example.proxy.ProxyInvocationHandler;
5
6 import java.util.concurrent.TimeUnit;
7
8 public class Person implements IPerson{
9
10 public static IPerson createPerson(){
11 Person person = new Person();
12 //TODO: 此处可以对Person对象进行额外的初始化工作
13 return (IPerson) ProxyInvocationHandler.getProxy(person);
14 }
15 @Override
16 @MyLog(beginMsg = "starting eat", endMsg = "finishing eat")
17 public boolean eat(String food) {
18 System.out.println("the person is eating " + food);
19 return true;
20 }
21
22 @Override
23 public void sleep(int seconds) {
24 System.out.println("the person is sleeping...");
25 try {
26 TimeUnit.SECONDS.sleep(seconds);
27 }catch (InterruptedException e){
28 e.printStackTrace();
29 }
30 }
31
32 @Override
33 @MyLog
34 public String walk(String start, String terminal) {
35 System.out.println("the person is walking from " + start + " to " + terminal);
36 return start + "--" + terminal;
37 }
38 }
最后实现InvocationHandler接口：
1 package org.example.proxy;
2
3 import org.example.annotation.MyLog;
4
5 import java.lang.reflect.InvocationHandler;
6 import java.lang.reflect.Method;
7 import java.lang.reflect.Proxy;
8 import java.time.LocalDateTime;
9 import java.time.format.DateTimeFormatter;
10
11 public class ProxyInvocationHandler implements InvocationHandler {
12 private Object target;
13
14 public ProxyInvocationHandler(Object target){
15 this.target = target;
16 }
17
18 static boolean equalParamTypes(Class<?>[] params1, Class<?>[] params2) {
19 if (params1.length == params2.length) {
20 for (int i = 0; i < params1.length; i++) {
21 if (params1[i] != params2[i])
22 return false;
23 }
24 return true;
25 }
26 return false;
27 }
28
29 @Override
30 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
31 Object result;
32
33 MyLog myLog = null;
34 Method[] methodList = target.getClass().getDeclaredMethods();
35 for (Method m : methodList) {
36 if (m.getName().endsWith(method.getName()) &&
37 m.getReturnType().equals(method.getReturnType()) &&
38 equalParamTypes(m.getParameterTypes(), method.getParameterTypes())) {
39
40 myLog = m.getAnnotation(MyLog.class);
41 break;
42 }
43 }
44
45 if (myLog != null) {
46 System.out.println(myLog.beginMsg() + LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
47 result = method.invoke(target, args);
48 System.out.println(myLog.endMsg() + LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
49 } else {
50 result = method.invoke(target, args);
51 }
52 return result;
53 }
54
55 public static Object getProxy(Object target){
56 return Proxy.newProxyInstance(
57 target.getClass().getClassLoader(),
58 target.getClass().getInterfaces(),
59 new ProxyInvocationHandler(target)
60 );
61 }
62
63 }
在Main中测试：
1 package org.example;
2
3 import org.example.business.IPerson;
4 import org.example.business.Person;
5
6 public class Main {
7 public static void main(String[] args) {
8
9 IPerson person = Person.createPerson();
10 person.eat("rice");
11 person.sleep(1);
12 String s = person.walk("shanghai", "beijing");
13 System.out.println(s);
14 }
15 }
输出结果如下：
starting eat2022-12-28T13:23:05.1305902
the person is eating rice
finishing eat2022-12-28T13:23:05.13759
the person is sleeping...
执行开始...2022-12-28T13:23:06.139794
the person is walking from shanghai to beijing
执行结束！！！2022-12-28T13:23:06.1487947
shanghai--beijing
方法二：使用CGLiB动态代理。
导入CGLiB依赖：
<dependency>
<groupId>cglib</groupId>
<artifactId>cglib</artifactId>
<version>3.3.0</version>
</dependency>
由于 JDK 8 中有关反射相关的功能自从 JDK 9 开始就已经被限制了，需要 Add VM options:
--add-opens java.base/java.lang=ALL-UNNAMED
来开启这种默认不被允许的行为。
定义一个普通类，该类可以不实现某个接口，注意该类的工厂方法createHuman()：
1 package org.example.business;
2
3 import org.example.annotation.MyLog;
4 import org.example.proxy.MyLogMethodInterceptor;
5
6 import java.util.concurrent.TimeUnit;
7
8 public class Human {
9
10 // 由newHuman()创建的对象才能使@MyLog生效
11 public static Human createHuman(){
12 Human human = (Human) MyLogMethodInterceptor.getProxy(Human.class);
13 //TODO: 此处可以对Human对象进行额外的初始化工作
14 return human;
15 }
16
17 @MyLog(beginMsg = "starting eat", endMsg = "finishing eat")
18 public boolean eat(String food) {
19 System.out.println("the person is eating " + food);
20 return true;
21 }
22
23 public void sleep(int seconds) {
24 System.out.println("the person is sleeping...");
25 try {
26 TimeUnit.SECONDS.sleep(seconds);
27 }catch (InterruptedException e){
28 e.printStackTrace();
29 }
30 }
31
32 @MyLog
33 public String walk(String start, String terminal) {
34 System.out.println("the person is walking from " + start + " to " + terminal);
35 return start + "--" + terminal;
36 }
37 }
最后实现MethodInterceptor接口：
1 package org.example.proxy;
2
3 import net.sf.cglib.proxy.Enhancer;
4 import net.sf.cglib.proxy.MethodInterceptor;
5 import net.sf.cglib.proxy.MethodProxy;
6 import org.example.annotation.MyLog;
7
8 import java.lang.reflect.Method;
9 import java.time.LocalDateTime;
10 import java.time.format.DateTimeFormatter;
11
12 public class MyLogMethodInterceptor implements MethodInterceptor {
13 @Override
14 public Object intercept(Object target, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
15 Object result;
16
17 MyLog myLog = method.getAnnotation(MyLog.class);
18 if (myLog != null) {
19 System.out.println(myLog.beginMsg() + LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
20 result = methodProxy.invokeSuper(target, args);
21 System.out.println(myLog.endMsg() + LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
22 } else {
23 result = methodProxy.invokeSuper(target, args);
24 }
25 return result;
26 }
27
28 public static Object getProxy(Class<?> clazz){
29 Enhancer enhancer = new Enhancer();
30 enhancer.setClassLoader(clazz.getClassLoader());
31 enhancer.setSuperclass(clazz);
32 enhancer.setCallback(new MyLogMethodInterceptor());
33 return enhancer.create();
34 }
35
36 }
在Main中测试：
1 package org.example;
2
3 import org.example.business.Human;
4
5 public class Main {
6 public static void main(String[] args) {
7
8 Human human = Human.createHuman();
9 human.eat("bread");
10 human.sleep(1);
11 String t = human.walk("杭州", "合肥");
12 System.out.println(t);
13 }
14 }
输出结果如下：
starting eat2022-12-28T13:36:19.0685967
the person is eating bread
finishing eat2022-12-28T13:36:19.0925967
the person is sleeping...
执行开始...2022-12-28T13:36:20.1026827
the person is walking from 杭州 to 合肥
执行结束！！！2022-12-28T13:36:20.1116888
杭州--合肥