爪哇 - 反射

简介

反射是爪哇的一种机制，是指在运行时，可以动态获取信息，可以动态调用对象方法，例如类、方法、接口、变量等信息（无需知道类的名字、方法名等）。

与爪哇反射相关的类如下：

类名	用途
Class 类	代表类的实体，在运行的爪哇应用程序中表示类和接口
Field 类	代表类的成员变量（成员变量也称为类的属性）
Method 类	代表类的方法
Constructor 类	代表类的构造方法

1. 获取一个将要被反射的类的方法：

1) 使用 .class 可以在编译时知道具体类名后，获取对象

Class studentClass = Student.class;

2) 使用 Class.forName("……") 在运行时获取对象。由于会找不到类，所以要进行异常捕获

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

3) 使用对象的 getClass() 方法

String s = new String("Hello");
Class sClass = s.getClass();

案例

package com.github.Student
public 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. 相关类说明

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()) + " 构造参数：");
    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 构造参数：[java.lang.String][int]

Process finished with exit code 0

II. 获取所有 public 方法

getConstructors() 可以获取以 public 修饰的所有构造方法

public class Test {
    public static void main(String[] args) {
        Class studentClass = Student.class;
        Constructor[] constructors = studentClass.getConstructors();
    }
}

III. 获取特定方法

通过向 getDelaredConstructor(……) 方法传参可以获得有着某些特定参数的方法，要进行异常捕获因为可能找不到指定的方法

Class[] classes = {String.class, int.class};
try {
    Class studentClass = Student.class;
    Constructor constructor = studentClass.getConstructor(classes);
    System.out.print(Modifier.toString(constructor.getModifiers()) + " 构造参数：");
    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 构造参数：[java.lang.String][int]

Process finished with exit code 0

3. 实际运用

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. 调用私有构造方法

Class[] classes = {String.class};
Class studentClass = Student.class;
Constructor constructor = studentClass.getDeclaredConstructor(classes);
constructor.setAccessible(true);
Student student = (Student) constructor.newInstance("Qwdfk", 19);

III. 调用方法

// 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}
您刚才说了 Salute! 吗？

Process finished with exit code 0

IV. 获取私有字段，并修改值

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