[Spring]为什么Spring动态代理默认使用CGlib，而不是JDK代理？

原因一：CGlib不需要接口

Spring动态代理默认使用CGlib，是因为它可以代理那些没有实现任何接口的类，而JDK动态代理仅能代理实现了接口的类。

原因二：CGlib效率高

CGlib相对于JDK动态代理来说，在代理类的创建和执行的速度上更快，因此在某些情况下，使用CGlib代理可以提高系统性能。

原因三：JDK代理会导致注解失效

如果Spring是JDK代理，那么就会导致某些注解失效。
因

如果希望使用JDK代理

• Spring可以设置proxyTargetClass属性为false来强制使用JDK代理。
• SpringBoot的AOP 默认使用 cglib，且无法通过proxyTargetClass进行修改。
  如果想修改的话，在Spring配置文件中添加spring.aop.proxy-target-class=false。

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扩展

AOP in Spring Boot, is it a JDK dynamic proxy or a Cglib dynamic proxy?

As we all know, the underlying AOP is dynamic proxies, and there are two ways to implement dynamic proxies in Java:

• JDK-based dynamic proxy
• Dynamic proxy based on Cglib

The biggest difference between these two is that JDK-based dynamic proxies require the object being proxied to implement an interface, while Cglib-based dynamic proxies do not require the object being proxied to implement an interface.

So, how is AOP implemented in Spring? Is it a dynamic proxy based on JDK or a dynamic proxy based on Cglib?

Spring

Let’s start with the conclusion that dynamic proxies in Spring, which one to use, are divided into cases.

If the proxy object implements the interface, then use the JDK dynamic proxy, otherwise it is the Cglib dynamic proxy.
If the proxy object does not implement an interface, then it is a direct Cglib dynamic proxy.

SpringBoot

Spring Boot and Spring are the same, so is it the same strategy for dynamic proxies? Sorry, it’s not really the same.

The handling of this issue in Spring Boot, with Spring Boot 2.0 as the node, is not the same.

Before Spring Boot 2.0, the code for automating the configuration of Aop looked like this (Spring Boot 1.5.22.RELEASE)

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = false)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false",
			matchIfMissing = true)
	public static class JdkDynamicAutoProxyConfiguration {

	}

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = true)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
			matchIfMissing = false)
	public static class CglibAutoProxyConfiguration {

	}

}
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As you can see, this automation configuration is mainly discussing the value of the spring.aop.proxy-target-class property in the application.properties configuration file.

The @ConditionalOnProperty annotation is what does the trick. To illustrate a few of the properties in this annotation.

• prefix: The prefix of the configuration file.
• name: the name of the configuration file, and prefix together form the key of the configuration.
• having: the value of the expected configuration. If the actual configuration is the same as the value of having, then the configuration will take effect, otherwise it will not.
• matchIfMissing: if the developer did not configure it in application.properties, then this configuration class will take effect or not.

Based on the introduction as above, it is easy to see that.

• If the developer has set spring.aop.proxy-target-class to false, then the JDK proxy is used.
• If the developer has spring.aop.proxy-target-class set to true, then the Cglib proxy is used.
  = If the developer did not configure the spring.aop.proxy-target-class property in the first place, then the JDK proxy is used.
  This was the case before Spring Boot 2.0.

Let’s look at the situation after Spring Boot 2.0 (inclusive) (Spring Boot 2.0.0.RELEASE).

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class,
		AnnotatedElement.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = false)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)
	public static class JdkDynamicAutoProxyConfiguration {

	}

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = true)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)
	public static class CglibAutoProxyConfiguration {

	}

}
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As you can see, most of the configuration is the same, with one area that is not quite the same, and that is the value of the matchIfMissing property.
As you can see, starting with Spring Boot 2.0, if the user does not configure anything, the Cglib proxy is used by default.

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