在这篇文章中，我们将揭秘 Spring 中面向方面编程（AOP）的内部机制。重点将放在理解 AOP 如何实现日志记录等功能，这些功能通常被认为是一种“魔法”。通过浏览核心 Java 实现，我们将了解 Java 的反射、代理模式和注释，而不是任何真正神奇的东西。
这些都是 java.lang.reflect、java.lang.annotation 和 javassist.util.proxy 包的一部分。
Spring AOP 的核心是代理对象、方法拦截器和反射的概念。此模式中的关键角色是 MethodHandler（或调用处理程序）。该处理程序通过拦截方法调用来控制代理对象的行为。当在代理上调用方法时，它会通过处理程序传递，其中可以通过反射对注释进行内省。根据应用的注释，可以在异常之前、之后或异常时执行必要的逻辑（例如日志记录）。
示例： 假设我们想在某些方法执行之前和之后添加日志记录。我们可以使用 @BeforeMethod 和 @AfterMethod 注释方法，而不是到处硬编码日志记录。我们的处理程序检查此注释的方法并动态添加适当的日志记录逻辑。
下面是我们示例中控制器和服务的类。
WorkerController.java
package edu.pk.poc.aop.controller;
import edu.pk.poc.aop.annotation.AfterMethod;
import edu.pk.poc.aop.annotation.All;
import edu.pk.poc.aop.annotation.BeforeMethod;
import edu.pk.poc.aop.helper.ProxyFactory;
import edu.pk.poc.aop.service.Worker;
import edu.pk.poc.aop.service.WorkerService;
import edu.pk.poc.aop.service.WorkerServiceImpl;
public class WorkerController {
WorkerService workerService = ProxyFactory.createProxy(WorkerServiceImpl.class);
/**
* This Method 1s annotated with @BeforeMethod and @AfterMethod, So the log statements
* will be generated before and after method call.
*/
@BeforeMethod
@AfterMethod
public void engageFullTimeWorker() throws Exception {
Worker fullTimeWorker = new Worker();
fullTimeWorker.setName("FullTime-Worker");
fullTimeWorker.setPartTime(false);
fullTimeWorker.setDuration(9);
workerService.doWork(fullTimeWorker);
}
/**
* This Method is annotated with @All, So the log statements will be generated before and after method call
* along with exception if raised.
*/
@All
public void engagePartTimeWorker() throws Exception {
Worker partTimeWorker = new Worker();
partTimeWorker.setName("PartTime-Worker");
partTimeWorker.setPartTime(true);
partTimeWorker.setDuration(4);
workerService.doWork(partTimeWorker);
}
}
WorkerServiceImpl.java
package edu.pk.poc.aop.service;
import edu.pk.poc.aop.annotation.AfterMethod;
public class WorkerServiceImpl implements WorkerService {
/**
* Here this method is annotated with only @AfterMethod, So only log statement
* will be generated after method call
*/
@AfterMethod
@Override
public void doWork(Worker worker) throws Exception {
if (worker.isPartTime()) {
throw new Exception("Part time workers are not permitted to work.");
}
System.out.print("A full time worker is working for " + worker.getDuration() + " hours :: ");
for (int i = 1; i < worker.getDuration(); i++) {
System.out.print("* ");
}
System.out.println();
}
}
Main.java 测试类
package edu.pk.poc.aop.test;
import edu.pk.poc.aop.controller.WorkerController;
import edu.pk.poc.aop.helper.ProxyFactory;
import edu.pk.util.Logger;
public class Main {
public static void main(String[] args) {
WorkerController controller = ProxyFactory.createProxy(WorkerController.class);
Logger logger = new Logger();
try {
System.out.println("Testing @BeforeMethod and @AfterMethod");
System.out.println("-----------------------------------------");
controller.engageFullTimeWorker();
System.out.println("Testing @All");
System.out.println("-----------------------------------------");
controller.engagePartTimeWorker();
} catch (Exception e) {
logger.error("Exception caught in Main class");
}
}
}
输出
Testing @BeforeMethod and @AfterMethod ----------------------------------------- >>> Entering into edu.pk.poc.aop.controller.WorkerController.engageFullTimeWorker() A full time worker is working for 9 hours :: * * * * * * * * >>> Exiting from edu.pk.poc.aop.service.WorkerServiceImpl.doWork() >>> Exiting from edu.pk.poc.aop.controller.WorkerController.engageFullTimeWorker() Testing @All ----------------------------------------- >>> Entering into edu.pk.poc.aop.controller.WorkerController.engagePartTimeWorker() >>> Exception in edu.pk.poc.aop.controller.WorkerController.engagePartTimeWorker() Exception caught in Main class
当在代理对象上调用方法时，该调用会被处理程序拦截，处理程序使用反射来检查目标方法上的所有注释。根据这些注释，处理程序决定是否记录方法进入/退出、记录异常或完全跳过记录。
以下是可视化它的方法：
定义注释
package edu.pk.poc.aop.annotation;
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.METHOD)
public @interface AfterMethod {
}
package edu.pk.poc.aop.annotation;
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.METHOD)
public @interface BeforeMethod {
}
package edu.pk.poc.aop.annotation;
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.METHOD)
public @interface All {
}
定义代理工厂
package edu.pk.poc.aop.helper;
/**
* The {@code ProxyFactory} class is responsible for creating proxy objects using the Javassist library.
* It allows for dynamic generation of proxies for classes or interfaces, with support for method interception.
*/
public class ProxyFactory {
/**
* A Javassist ProxyFactory instance used to generate proxy classes.
*/
private static final javassist.util.proxy.ProxyFactory factory = new javassist.util.proxy.ProxyFactory();
/**
* Creates a proxy object for the given class or interface.
* If the class is an interface, the proxy implements the interface.
* If it's a concrete class, the proxy extends the class.
*
* @param <T> the type of the class or interface for which the proxy is to be created
* @param klass the {@code Class} object representing the class or interface to proxy
* @return a proxy instance of the specified class or interface, or {@code null} if proxy creation fails
*/
public static <T> T createProxy(Class<T> klass) {
if (klass.isInterface())
factory.setInterfaces(new Class[]{klass});
else
factory.setSuperclass(klass);
try {
return (T) factory.create(new Class<?>[0], new Object[0], new AOPLoggingMethodHandler());
} catch (Exception e) {
System.err.println(e.getMessage());
}
return null;
}
}
定义 MethodHandler
package edu.pk.poc.aop.helper;
import edu.pk.poc.aop.annotation.AfterMethod;
import edu.pk.poc.aop.annotation.All;
import edu.pk.poc.aop.annotation.BeforeMethod;
import edu.pk.poc.aop.annotation.OnException;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import edu.pk.util.Logger;
import javassist.util.proxy.MethodHandler;
public class AOPLoggingMethodHandler implements MethodHandler {
private static final Logger logger = new Logger();
public Object invoke(Object self, Method thisMethod, Method proceed, Object[] args) throws Throwable {
if (proceed != null) { // Concrete Method
Object result = null;
String className = resolveClassName(self);
try {
if (isAnnotationPresent(thisMethod, BeforeMethod.class) || isAnnotationPresent(thisMethod, All.class)) {
logger.info(">>> Entering into " + className + "." + thisMethod.getName() + "()");
}
result = proceed.invoke(self, args);
if (isAnnotationPresent(thisMethod, AfterMethod.class) || isAnnotationPresent(thisMethod, All.class)) {
logger.info(">>> Exiting from " + className + "." + thisMethod.getName() + "()");
}
} catch (Throwable t) {
if (isAnnotationPresent(thisMethod, OnException.class) || isAnnotationPresent(thisMethod, All.class)) {
logger.error(">>> Exception in " + className + "." + thisMethod.getName() + "()");
}
throw t;
}
return result;
}
throw new RuntimeException("Method is Abstract");
}
private boolean isAnnotationPresent(Method method, Class klass) {
Annotation[] declaredAnnotationsByType = method.getAnnotationsByType(klass);
return declaredAnnotationsByType != null && declaredAnnotationsByType.length > 0;
}
private String resolveClassName(Object self) {
String className = self.getClass().getName();
if (className.contains("_$$")) {
className = className.substring(0, className.indexOf("_$$"));
}
return className;
}
}
Spring AOP 是一个用于横切关注点的强大工具，但它并没有做任何革命性的事情。它建立在反射和代理等核心 Java 概念之上，这些概念在语言本身中可用。通过理解这一点，您可以更好地理解 Spring 如何简化这些底层机制以方便开发人员。
以上是揭示 Spring AOP 的内部工作原理的详细内容。更多信息请关注PHP中文网其他相关文章！
