Implementation of Java reflection mechanism in Java virtual machine?

The Java reflection mechanism is implemented in the Java Virtual Machine (JVM) through the Class object, which contains metadata information about the class. The Reflection API provides classes and interfaces such as Class, Field, Method, and Constructor for accessing this information. The reflection mechanism allows obtaining class information (such as class name, fields and methods), obtaining field information (such as type and name), obtaining method information (such as return type and parameter type), and modifying object state (such as modifying private fields) at runtime. value).

Implementation of Java reflection mechanism in Java Virtual Machine (JVM)

Introduction

The Java reflection mechanism is a powerful and flexible feature that allows a program to inspect or modify the structure and behavior of a class at runtime. This article will delve into the implementation of Java reflection in the JVM and explain it through practical cases.

Class Object

The Java Virtual Machine creates a Class object for each loaded class. The Class object contains metadata information about a class, such as its name, fields, methods, and constructors.

Reflection API

The reflection API provides a set of classes and interfaces for accessing metadata information of Class objects. The most commonly used classes include:

• Class: Represents a class.
• Field: Represents a field in a class.
• Method: Represents a method in a class.
• Constructor: Represents the constructor of a class.

Practical case

Get class information

// 获取 Class 对象
Class<?> cls = Class.forName("java.lang.String");

// 获取类名
System.out.println("类名：" + cls.getName());

// 获取类访问修饰符
System.out.println("类访问修饰符：" + Modifier.toString(cls.getModifiers()));

Get field information

// 获取指定字段的 Field 对象
Field field = cls.getDeclaredField("value");

// 获取字段类型
System.out.println("字段类型：" + field.getType().getName());

// 获取字段名称
System.out.println("字段名称：" + field.getName());

Get method information

// 获取指定方法的 Method 对象
Method method = cls.getMethod("length");

// 获取方法返回类型
System.out.println("方法返回类型：" + method.getReturnType().getName());

// 获取方法参数列表
Class<?>[] paramTypes = method.getParameterTypes();
for (Class<?> paramType : paramTypes) {
    System.out.println("方法参数类型：" + paramType.getName());
}

Modify object status

The reflection mechanism can also be used to modify the status of the object.

// 获取一个 String 对象
String str = "Hello World";

// 获取 String 对象的 Class 对象
Class<?> cls = str.getClass();

// 获取字段对象
Field field = cls.getDeclaredField("value");

// 设置字段为可访问（私有字段）
field.setAccessible(true);

// 修改字符串值
field.set(str, "Goodbye World");

// 输出修改后的字符串
System.out.println(str); // 输出：Goodbye World

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