author ： Little brother Fu
Blog ：https://bugstack.cn

precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！

One 、 Preface

Programming learning , The width or the depth of digging first ？

In fact, the width and depth of technology complement each other , How much technology you can learn is related to the depth of your understanding of a technology , And how deep you can explore a technology requires you to have a certain breadth of cognition . Otherwise, if you just want to understand the skin or knock a piece of code , The harvest is not necessarily big , Or it costs a lot .

Technical bottlenecks , Age related or big factory ？

I've been an interviewer for a long time , I've interviewed a lot of people . Sometimes you don't have to be old enough to be skilled , It doesn't have to be just working 2 It's not going to work for about a year . Often we say some interviews make rockets , But in the answers of these job seekers , Can give very accurate answers . That is, he can answer to the point , This is what you can do with understanding .

Working hours and whether in a large factory , These are the amount of resources that can be accessed , See the height of technical insight . But I really want to absorb these things to myself , Still need individual struggle . Otherwise, a lot of things will be put in front of you , It's hard for you to see . Most of the time you can see it's just the title

Two 、 Interview questions

Thank you plane , Notes ,10.1 Holiday play hi plane , Seems to have set their own learning goals before the holiday ！ But the thought of another interview , I can't help cramming , I started reading brother Fu's blog ：bugstack.cn

interviewer ： The plane , Look at you flustered ？

Thank you plane ： No, , No, , I'm afraid I didn't have time to run upstairs .

interviewer ： good ！ I don't think your resume has been updated , Let's talk about dynamic proxy and reflection this time , Do you know how to proxy a class ？

Thank you plane ： I know that , Use JDK Own class Proxy You can delegate a class , You can also use CGLIB agent .

interviewer ： Um. , What's the difference between the two agents ？

Thank you plane ： Like one is JDK Need to have an interface ,CGLIB You don't need to .

interviewer ： Why? ？

Thank you plane ： Why? ？ this …

interviewer ： When you develop it yourself , Do you do any business with an agent ？

Thank you plane ：… It doesn't seem that ！

The plane can only slip home , Lack of technical depth , And it hasn't been used in practice , There is still a lot to be done ！

3、 ... and 、 Five types of agency

No accident , You may only know two types of agency . One is JDK Self contained , The other is CGLIB.

We first define an interface and the corresponding implementation class , It is convenient for subsequent use of proxy classes to add output information in methods .

Defining interfaces

public interface IUserApi {
String queryUserInfo();
}

Implementation interface

public class UserApi implements IUserApi {
public String queryUserInfo() {
return " Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！";
}
}

good ！ Next, we'll add a line of extra output information to the class method using the proxy .

0. First, add a little knowledge of reflection

@Test
public void test_reflect() throws Exception {
Class<UserApi> clazz = UserApi.class;
Method queryUserInfo = clazz.getMethod("queryUserInfo");
Object invoke = queryUserInfo.invoke(clazz.newInstance());
System.out.println(invoke);
}

• Index ：
• comment ： Where there's a proxy, there's almost a reflection , They are a set of functional classes that work together . Methods can be called in reflection 、 get attribute 、 Get the notes, and so on . These can be combined with the following class proxy , Complete the technical scenarios in various frameworks .

1. JDK Agency mode

public class JDKProxy {
public static <T> T getProxy(Class clazz) throws Exception {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
return (T) Proxy.newProxyInstance(classLoader, new Class[]{clazz}, new InvocationHandler() {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println(method.getName() + " You have been represented ,By JDKProxy！");
return " Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！";
}
});
}
}
@Test
public void test_JDKProxy() throws Exception {
IUserApi userApi = JDKProxy.getProxy(IUserApi.class);
String invoke = userApi.queryUserInfo();
logger.info(" test result ：{}", invoke);
}
/**
* test result ：
*
* queryUserInfo You have been represented ,By JDKProxy！
* 19:55:47.319 [main] INFO org.itstack.interview.test.ApiTest - test result ： Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！
*
* Process finished with exit code 0
*/

• Index ：
• scene ： Middleware development 、 The application of agent pattern and decorator pattern in design pattern
• comment ： such JDK The built-in class proxy is a very common one , It's also a very simple one . You can see it in some middleware code, such as ： Database routing component 、Redis Components etc. , At the same time, we can also apply it to design patterns in this way .

2. CGLIB Agency mode

public class CglibProxy implements MethodInterceptor {
public Object newInstall(Object object) {
return Enhancer.create(object.getClass(), this);
}
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
System.out.println(" I was CglibProxy Agent ");
return methodProxy.invokeSuper(o, objects);
}
}
@Test
public void test_CglibProxy() throws Exception {
CglibProxy cglibProxy = new CglibProxy();
UserApi userApi = (UserApi) cglibProxy.newInstall(new UserApi());
String invoke = userApi.queryUserInfo();
logger.info(" test result ：{}", invoke);
}
/**
* test result ：
*
* queryUserInfo You have been represented ,By CglibProxy！
* 19:55:47.319 [main] INFO org.itstack.interview.test.ApiTest - test result ： Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！
*
* Process finished with exit code 0
*/

• Index ：
• scene ：Spring、AOP section 、 Authentication services 、 Middleware development 、RPC Frame, etc
• comment ：CGLIB differ JDK, Its underlying use ASM Bytecode framework modifies the script implementation agent in a class , So this kind of agency doesn't need to be like JDK That requires an interface to proxy . At the same time, thanks to the use of bytecode framework , So this kind of proxy is better than using JDK The way of agency is fast 1.5~2.0 times .

3. ASM Agency mode

public class ASMProxy extends ClassLoader {
public static <T> T getProxy(Class clazz) throws Exception {
ClassReader classReader = new ClassReader(clazz.getName());
ClassWriter classWriter = new ClassWriter(classReader, ClassWriter.COMPUTE_MAXS);
classReader.accept(new ClassVisitor(ASM5, classWriter) {
@Override
public MethodVisitor visitMethod(int access, final String name, String descriptor, String signature, String[] exceptions) {
// Method filtration 
if (!"queryUserInfo".equals(name))
return super.visitMethod(access, name, descriptor, signature, exceptions);
final MethodVisitor methodVisitor = super.visitMethod(access, name, descriptor, signature, exceptions);
return new AdviceAdapter(ASM5, methodVisitor, access, name, descriptor) {
@Override
protected void onMethodEnter() {
// Execution instruction ; Get static properties 
methodVisitor.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
// Load constant load constant
methodVisitor.visitLdcInsn(name + " You have been represented ,By ASM！");
// Calling method 
methodVisitor.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
super.onMethodEnter();
}
};
}
}, ClassReader.EXPAND_FRAMES);
byte[] bytes = classWriter.toByteArray();
return (T) new ASMProxy().defineClass(clazz.getName(), bytes, 0, bytes.length).newInstance();
}
}
@Test
public void test_ASMProxy() throws Exception {
IUserApi userApi = ASMProxy.getProxy(UserApi.class);
String invoke = userApi.queryUserInfo();
logger.info(" test result ：{}", invoke);
}
/**
* test result ：
*
* queryUserInfo You have been represented ,By ASM！
* 20:12:26.791 [main] INFO org.itstack.interview.test.ApiTest - test result ： Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！
*
* Process finished with exit code 0
*/

• Index ：
• scene ： Full link monitoring 、 Crack toolkit 、CGLIB、Spring Get class metadata, etc
• comment ： This kind of proxy is to use bytecode programming for processing , Its implementation is relatively complex , And you need to understand Java Knowledge of virtual machine specification . Because every step of your proxy operation , They're all manipulating bytecode instructions , for example ：Opcodes.GETSTATIC、Opcodes.INVOKEVIRTUAL, In addition to these, there are small 200 A common command . But this is the closest way to the bottom , It's also the fastest way . So in some full link monitoring using bytecode instrumentation , It's very common .

4. Byte-Buddy Agency mode

public class ByteBuddyProxy {
public static <T> T getProxy(Class clazz) throws Exception {
DynamicType.Unloaded<?> dynamicType = new ByteBuddy()
.subclass(clazz)
.method(ElementMatchers.<MethodDescription>named("queryUserInfo"))
.intercept(MethodDelegation.to(InvocationHandler.class))
.make();
return (T) dynamicType.load(Thread.currentThread().getContextClassLoader()).getLoaded().newInstance();
}
}
@RuntimeType
public static Object intercept(@Origin Method method, @AllArguments Object[] args, @SuperCall Callable<?> callable) throws Exception {
System.out.println(method.getName() + " You have been represented ,By Byte-Buddy！");
return callable.call();
}
@Test
public void test_ByteBuddyProxy() throws Exception {
IUserApi userApi = ByteBuddyProxy.getProxy(UserApi.class);
String invoke = userApi.queryUserInfo();
logger.info(" test result ：{}", invoke);
}
/**
* test result ：
*
* queryUserInfo You have been represented ,By Byte-Buddy！
* 20:19:44.498 [main] INFO org.itstack.interview.test.ApiTest - test result ： Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！
*
* Process finished with exit code 0
*/

• Index ：
• scene ：AOP section 、 Class agent 、 Components 、 monitor 、 journal
• comment ：Byte Buddy It is also a bytecode operation class library , but Byte Buddy It's easier to use . No need to understand bytecode instructions , You can use the simple API It's easy to manipulate bytecode , Control classes and methods . Compared with JDK A dynamic proxy 、cglib,Byte Buddy It has some advantages in performance . in addition ,2015 year 10 month ,Byte Buddy By Oracle Granted Duke’s Choice Awards . The award is for Byte Buddy Of “ Java Great innovation in technology ” Express appreciation for .

5. Javassist Agency mode

public class JavassistProxy extends ClassLoader {
public static <T> T getProxy(Class clazz) throws Exception {
ClassPool pool = ClassPool.getDefault();
// Get class 
CtClass ctClass = pool.get(clazz.getName());
// Access method 
CtMethod ctMethod = ctClass.getDeclaredMethod("queryUserInfo");
// Pre method strengthening 
ctMethod.insertBefore("{System.out.println(\"" + ctMethod.getName() + " You have been represented ,By Javassist\");}");
byte[] bytes = ctClass.toBytecode();
return (T) new JavassistProxy().defineClass(clazz.getName(), bytes, 0, bytes.length).newInstance();
}
}
@Test
public void test_JavassistProxy() throws Exception {
IUserApi userApi = JavassistProxy.getProxy(UserApi.class)
String invoke = userApi.queryUserInfo();
logger.info(" test result ：{}", invoke);
}
/**
* test result ：
*
* queryUserInfo You have been represented ,By Javassist
* 20:23:39.139 [main] INFO org.itstack.interview.test.ApiTest - test result ： Little brother Fu , official account ：bugstack Wormhole stack | precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！
*
* Process finished with exit code 0
*/

• Index ：
• scene ： Full link monitoring 、 Class agent 、AOP
• comment ：Javassist Is a very widely used bytecode instrumentation framework , Almost a majority of non intrusive full link monitoring will choose to use this framework . Because it doesn't want to ASM Operating bytecode that way leads to risk , At the same time, its functions are very complete . in addition , This framework can write instrumentation code directly in the way it provides , You can also use bytecode instructions to control the generation of code , So overall, it's a very good bytecode framework .

Four 、 summary

• The actual purpose of agency is through some technical means , Replace the original implementation class or inject new bytecode instructions into the original implementation class . And these technologies will eventually use some framework applications 、 Middleware development and similar non-invasive full link monitoring .
• A deep study of technology stack can let you thoroughly understand some basic fundamental principles , Through this kind of study, we can get rid of some vague questions , Also can let oneself have better job opportunity and salary through the development of this kind of Technology .
• These technologies are not easy to learn , There may even be some brain burn . But every piece of technology worth learning can help you break through the technical bottleneck at a certain stage .

5、 ... and 、 Series recommendation

• Bytecode Programming Series topic
• HashMap Core knowledge , Disturbance function 、 Load factor 、 Split the expanded list
• ThreadLocal Technology stack deep learning
• Is the grass , You're poisoning the code ！

bugstack Wormhole stack

precipitation 、 Share 、 grow up , Let yourself and others have something to gain

Wechat scan QR code , Follow my public number

（ Please indicate the author and source of this article WeChat official account ：bugstack Wormhole stack | author ： Little brother Fu ）