Byte code in the Java class library: the principle and purpose of implementation
The bytecode in the Java class library: implementation principle and use
Summary:
The byte code is the middle form of the Java language. The Java compiler compiles the source code into bytecode and is executed on the Java virtual machine (JVM).Bytecodes are widely used in the Java library, and they are the basic components of the Java program function.This article will introduce the principles and application scenarios of bytecode, and provide some Java code examples to help readers understand the concept of bytecode.
1 Introduction
Java bytecode is a binary format generated by the Java program after compiling.It has nothing to do with the platform and can be performed on any platform that supports Java virtual machines.The byte code is composed of a series of instructions, which are interpreted and executed by Java virtual machines one by one.In order to better geographical understanding the principles and application scenarios of bytecode, detailed discussion will be discussed below.
2. The principle of bytecode implementation
After the Java source code is processed by the compiler, it will be converted into bytecode.The byte code uses a stack instruction format, which means that all operations are based on the stack.Each Java bytecode instruction is usually composed of an operating code and the corresponding operating number.These instructions can perform various operations, such as loading, storage, computing, control flow, etc.
The implementation principle of bytecode is based on the specifications of the Java virtual machine. It defines the instruction set and execution model of the Java virtual machine.Java virtual machine executes the Java program by explaining bytecode instructions.It provides two execution methods: bytecode interpreter and instantaneous compiler. Among them, the byte code interpreter explains the byte code instruction one by one, and the instant code converts the byte code into a local machine code to improve execution efficiency.
3. Application scenario of bytecode
The byte code has a wide range of application scenarios in the Java class library. The following lists are listed several common application examples:
3.1 Reflection (Reflection):
The reflection mechanism of the Java program can be implemented through bytecode.The reflection allows the program to dynamically obtain and operate the information of the class during runtime, such as the method, field, constructor, etc.By analyzing the byte code, the program can realize the functions of dynamic creation objects, calling methods, and modification of the field values.
Example code:
Class<?> clazz = Class.forName("com.example.MyClass");
Object obj = clazz.newInstance();
Method method = clazz.getMethod("myMethod");
method.invoke(obj);
3.2 Dynamic Proxy:
Bytecodes are also widely used in the dynamic proxy mode of implementing Java.The dynamic proxy allows the program to generate the proxy class during runtime, and the proxy class can intercept and process the target object.The byte code generation tool can dynamically generate proxy classes and call the target object method to the proxy class processing.
Example code:
public interface MyInterface {
void myMethod();
}
public class MyProxy implements InvocationHandler {
private Object target;
public MyProxy(Object target) {
this.target = target;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// do something before method invocation
Object result = method.invoke(target, args);
// do something after method invocation
return result;
}
}
MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
MyInterface.class.getClassLoader(),
new Class<?>[] { MyInterface.class },
new MyProxy(new MyObject())
);
proxy.myMethod();
3.3 AOP（Aspect-Oriented Programming）：
AOP is a programming paradigm. It separates cross -cutting points from the core business logic to improve the maintenance and reuse of code.Bytecode can realize the underlying mechanism of AOP, such as bytecode -based interceptors, dynamic injection, etc.Through bytecode operating library, the byte code can be modified during the compilation period or runtime to implement the AOP function.
Example code:
@Aspect
public class MyAspect {
@Before("execution(void com.example.MyClass.myMethod())")
public void beforeAdvice() {
System.out.println("Before method execution.");
}
}
@AnnotatedClass
public class MyClass {
public void myMethod() {
System.out.println("My method execution.");
}
}
AspectJProxyFactory factory = new AspectJProxyFactory(new MyClass());
factory.addAspect(new MyAspect());
MyClass proxy = factory.getProxy();
proxy.myMethod();
4. Summary
This article introduces the implementation principles and application scenarios in the JAVA class library.As the middle form of the Java program, the byte code plays an important role in many fields, such as reflection, dynamic proxy, AOP, etc.It is hoped that through the introduction of this article, readers have a deeper understanding of bytecode and can be better applied to actual development.
references:
-"Java Virtual Machine Specification (Java SE 8 Edition)"
