揭示 Spring AOP 的内部工作原理
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。
先决条件
- java 核心代理 api
- 反射api
- 注释api
这些都是 java.lang.reflect、java.lang.annotation 和 javassist.util.proxy 包的一部分。
核心机制
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）。
分解它
- 代理对象：这些是动态创建的对象，代表您的实际业务对象，通过方法处理程序路由方法调用。
- 调用处理程序：这就是拦截的神奇之处。使用反射，处理程序可以检查目标方法上存在的注释并相应地改变行为。
- 自定义注释：您可以定义自定义注释，它们用作触发日志记录、安全检查或事务管理等附加功能的标记。
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
<p><strong>main.java 测试类</strong><br></p>
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
它是如何运作的
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;
}
}
</t></t></t>
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; } }
结论
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。
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