简介

反射是爪哇的一种机制，是指在运行时，能够动静获取信息，能够动静调用对象办法，例如类、办法、接口、变量等信息（无需晓得类的名字、办法名等）。

与爪哇反射相干的类如下：

类名用处
Class 类代表类的实体，在运行的爪哇应用程序中示意类和接口
Field 类代表类的成员变量（成员变量也称为类的属性）
Method 类代表类的办法
Constructor 类代表类的构造方法

1. 获取一个将要被反射的类的办法：

1) 应用 .class 能够在编译时晓得具体类名后，获取对象

Class studentClass = Student.class;

2) 应用 Class.forName("……") 在运行时获取对象。因为会找不到类，所以要进行异样捕捉

try {    Class studentClass = Class.forName("Student");} catch(ClassNotFoundException e) {    e.printStackTrace();}

3) 应用对象的 getClass() 办法

String s = new String("Hello");Class sClass = s.getClass();
案例
package com.github.Studentpublic class Test {    public static void main (String[] args) {        Class studentClass = Student.class;        System.out.println(studentClass.getName());        System.out.println(studentClass.getSimpleName());    }}

/Library/Java/JavaVirtualMachines/jdk-15.0.2.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=49728:/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/Code/works/out/production/works com.github.Student
com.github.Student
Student

Process finished with exit code 0

2. 相干类阐明

1) Class 类

2) Field 类

3) Method 类

4) Constructor 类

I. 获取所有构造方法
public class Test {    public static void main(String[] args) {        Class studentClass = Student.class;        Constructor[] constructors = studentClass.getDeclaredConstructors();    }}
附：获取所有构造方法以及每个构造方法的参数类型
for(int i = 0; i < constructors.length; i++) {    System.out.print(Modifier.toString(constructors[i].getModifiers()) + " 结构参数：");    Class[] classes = constructors[i].getParameterTypes();    for(int j = 0; j < classes.length; j++) {        System.out.print("[" + classes[j].getName() + "]");    }    System.out.println();}

/Library/Java/JavaVirtualMachines/jdk-15.0.2.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=52590:/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/Code/works/out/production/works com.github.Test
public 结构参数：[java.lang.String][int]

Process finished with exit code 0

II. 获取所有 public 办法

getConstructors() 能够获取以 public 润饰的所有构造方法

public class Test {    public static void main(String[] args) {        Class studentClass = Student.class;        Constructor[] constructors = studentClass.getConstructors();    }}
III. 获取特定办法

通过向 getDelaredConstructor(……) 办法传参能够取得有着某些特定参数的办法，要进行异样捕捉因为可能找不到指定的办法

Class[] classes = {String.class, int.class};try {    Class studentClass = Student.class;    Constructor constructor = studentClass.getConstructor(classes);    System.out.print(Modifier.toString(constructor.getModifiers()) + " 结构参数：");    Class[] parameters = constructor.getParameterTypes();    for(int j = 0; j < classes.length; j++) {        System.out.print("[" + classes[j].getName() + "]");    }    System.out.println();} catch(NoSuchMethodException e) {    e.printStackTrace();}

/Library/Java/JavaVirtualMachines/jdk-15.0.2.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=53473:/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/Code/works/out/production/works com.github.Test
public 结构参数：[java.lang.String][int]

Process finished with exit code 0

3. 理论使用

I. 调用构造方法
Class[] classes = {String.class, int.class};Class studentClass = Student.class;Constructor constructor = studentClass.getDeclaredConstructor(classes);// constructor.newInstance("Asetg", 10);Student student = (Student) constructor.newInstance("Ynioh", 19);
II. 调用公有构造方法
Class[] classes = {String.class};Class studentClass = Student.class;Constructor constructor = studentClass.getDeclaredConstructor(classes);constructor.setAccessible(true);Student student = (Student) constructor.newInstance("Qwdfk", 19);
III. 调用办法
// 获取无参无返回的 show()Method method = studentClass.getDeclaredMethod("show");method.invoke(constructor.newInstance("Asetg", 10));// 获取有参无返回的 repeat()Class[] parameters = {String.class};Method method2 = studentClass.getDeclaredMethod("repeat", parameters);Object[] objects = {" Salute! "};method2.invoke(constructor.newInstance("Asetg", 10), objects);

/Library/Java/JavaVirtualMachines/jdk-15.0.2.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=58735:/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/Code/works/out/production/works com.github.Test
Student{name='Asetg', age=10}
您方才说了 Salute! 吗？

Process finished with exit code 0

IV. 获取公有字段，并批改值
Field field = studentClass.getDeclaredField("name");field.setAccessible(true);field.set(student, "Zxcvb");System.out.println(field.get(student).toString());

/Library/Java/JavaVirtualMachines/jdk-15.0.2.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA CE.app/Contents/lib/idea_rt.jar=58823:/Applications/IntelliJ IDEA CE.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/Code/works/out/production/works com.github.Test
Zxcvb

Process finished with exit code 0