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This article focuses on "does Spring support Aop enhancements to static methods?" friends who are interested may wish to take a look. The method introduced in this paper is simple, fast and practical. Let the editor take you to learn "does Spring support Aop enhancements to static methods?"

1. JDK agent

JDK Agent Code:

Import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.lang.reflect.Proxy;interface Echor {public void echo ();} class EchorImpl implements Echor {@ Override public void echo () {System.out.println ("echo ~");}} class MethodInvoker implements InvocationHandler {private T invoker; public MethodInvoker (T invoker) {this.invoker = invoker } @ Override public Object invoke (Object proxy, Method method, Object [] args) throws Throwable {System.out.println ("start ~"); Object result = method.invoke (invoker, args); System.out.println ("end ~"); return result }} public class DebugJdkProxy {public static void main (String [] args) {Echor proxy = (Echor) Proxy.newProxyInstance (DebugJdkProxy.class.getClassLoader (), new Class [] {Echor.class}, new MethodInvoker (new EchorImpl (); proxy.echo ();}}

JVM implements sun.misc.ProxyGenerator, which is an important proxy class. The method of generating proxy class is generateClassFile source code:

Private byte [] generateClassFile () {this.addProxyMethod (hashCodeMethod, Object.class); this.addProxyMethod (equalsMethod, Object.class); this.addProxyMethod (toStringMethod, Object.class); Class [] var1 = this.interfaces; int var2 = var1.length; int var3; Class var4; for (var3 = 0; var3)

< var2; ++var3) { var4 = var1[var3]; //重点：代理那些方法？实例方法 Method[] var5 = var4.getMethods(); int var6 = var5.length; for(int var7 = 0; var7 < var6; ++var7) { Method var8 = var5[var7]; this.addProxyMethod(var8, var4); } } Iterator var11 = this.proxyMethods.values().iterator(); List var12; while(var11.hasNext()) { var12 = (List)var11.next(); checkReturnTypes(var12); } Iterator var15; try { this.methods.add(this.generateConstructor()); var11 = this.proxyMethods.values().iterator(); while(var11.hasNext()) { var12 = (List)var11.next(); var15 = var12.iterator(); while(var15.hasNext()) { ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next(); this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10)); this.methods.add(var16.generateMethod()); } } this.methods.add(this.generateStaticInitializer()); } catch (IOException var10) { throw new InternalError("unexpected I/O Exception", var10); } if (this.methods.size() >

65535) {throw new IllegalArgumentException ("method limit exceeded");} else if (this.fields.size () > 65535) {throw new IllegalArgumentException ("field limit exceeded");} else {this.cp.getClass (dotToSlash (this.className)); this.cp.getClass ("java/lang/reflect/Proxy"); var1 = this.interfaces; var2 = var1.length For (var3 = 0; var3)

< var2; ++var3) { var4 = var1[var3]; this.cp.getClass(dotToSlash(var4.getName())); } this.cp.setReadOnly(); ByteArrayOutputStream var13 = new ByteArrayOutputStream(); DataOutputStream var14 = new DataOutputStream(var13); try { var14.writeInt(-889275714); var14.writeShort(0); var14.writeShort(49); this.cp.write(var14); var14.writeShort(this.accessFlags); var14.writeShort(this.cp.getClass(dotToSlash(this.className))); var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy")); var14.writeShort(this.interfaces.length); Class[] var17 = this.interfaces; int var18 = var17.length; for(int var19 = 0; var19 < var18; ++var19) { Class var22 = var17[var19]; var14.writeShort(this.cp.getClass(dotToSlash(var22.getName()))); } var14.writeShort(this.fields.size()); var15 = this.fields.iterator(); while(var15.hasNext()) { ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next(); var20.write(var14); } var14.writeShort(this.methods.size()); var15 = this.methods.iterator(); while(var15.hasNext()) { ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next(); var21.write(var14); } var14.writeShort(0); return var13.toByteArray(); } catch (IOException var9) { throw new InternalError("unexpected I/O Exception", var9); } } } 上DEBUG截图：

At this point, it is clear which methods are proxied when the JDK underlying generates the proxy class, where reflection getMethods is that you can get all the public methods in the Class, including static methods.

Because the JDK proxy is based on the interface, and static methods are not allowed in the interface, static methods cannot be proxied. To put it another way: both interface-based Jdk proxies and inheritance-based Class-based proxies are essentially proxies that override the implementation of specified methods after inheritance, while static methods belong to class, not class instances, and cannot be overridden, so static methods cannot be proxied. In addition, the JDK proxy class is generated based on the interface implementation, so final methods for subclasses can be proxied.

It is important to note that the default mode in Jdk8 is an instance method and a static method.

2. CGLIB agent import net.sf.cglib.proxy.Enhancer;import net.sf.cglib.proxy.MethodInterceptor;import net.sf.cglib.proxy.MethodProxy; interface Echor {public void echo (); public static void hello () {System.out.println ("hello world!");}} abstract class AbsEchor implements Echor {public static void abs () {System.out.println ("abs~~") } public static void hello () {System.out.println ("hello world!");}} class EchorImpl implements Echor {public static void hello2 () {System.out.println ("hello world!");} @ Override public void echo () {System.out.println ("echo ~") }} class EchorMethodInterceptor implements MethodInterceptor {@ Override public Object intercept (Object obj, Method method, Object [] args, MethodProxy proxy) throws Throwable {System.out.println ("start ~"); Object result = proxy.invokeSuper (obj, args); System.out.println ("end ~"); return result;}} class DebugCGlibProxy {public static void main (String [] args) {Enhancer enhancer = new Enhancer () Enhancer.setSuperclass (AbsEchor.class); enhancer.setCallback (new EchorMethodInterceptor ()); AbsEchor hello = (AbsEchor) enhancer.create (); hello.abs ();}}

Summary: the essence of proxy class generation based on JDK proxy and CGLIB proxy is based on inheritance rewriting (the implementation interface can be regarded as a special inheritance); for static member methods, there is no way to subclass rewrite, static belongs to class.

So far: because Spring uses both JDK and CGLIB to implement AOP, the conclusion is that Spring cannot support the proxy enhancement of the static method.

Spring AOP static proxy

We should compare AOP with OOP. The differences between them are as follows:

What has to be mentioned in AOP is the agent.

The popular understanding is that Maotai produces wine, while agents take it out to sell and launch various sales activities. At this time, Maotai company is the real theme, that is, the target. And the agent is the agent. Maotai is the method in the target object. Various sales activities are enhancements to methods in the target object, such as adding logs to methods, and so on.

Agents are divided into static agents and dynamic agents: it is known that the target object is Maotai Company, and the target object has been determined.

Here is an example of a static agent

First define an interface for PersonBiz:

Then implement this interface.

This is our ability to add an operation time to these two methods using a static proxy, so I went straight to the code:

Package com.yc.dao.impl;import java.util.Date;import com.yc.dao.PersonBiz;// proxy object public class PersonBizProxy implements PersonBiz {private PersonBiz personBiz;// references to real topics public PersonBizProxy (PersonBiz personBiz) {this.personBiz = personBiz;} @ Override public void add (String name) {/ / add concerns-"enhanced features showLog () / / pre-enhancement / / then call the real theme method this.personBiz.add (name);} @ Override public String find () {/ / call the real theme method personBiz.find (); / / add concerns-"enhanced function showLog (); / / post-enhanced return null } private void showLog () {Date d = new Date (); System.out.println ("-"); System.out.println ("operating time" + d); System.out.println ("-");}}

Finally, there is the test class:

The advantage of the agent is obvious: when you do not need the new operation time function, you can PersonBizProxy pbp=new PersonBizProxy (pb); remove it, and then use the pb call method, so that the code is extensible and does not need to be modified on the existing code.

Disadvantages of static proxies: proxies can only be used for one interface

At this point, I believe you have a deeper understanding of "does Spring support Aop enhancements to static methods?" you might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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