I. Overview of reflection

Java’s reflection mechanism allows you to know all the properties and methods of any class at runtime. You can call any of its methods and properties for any object. This ability to dynamically retrieve information and invoke methods is called the Reflection mechanism of the Java language. Reflection encapsulates the components of a class into an object from which instances of the class can be created. = =

Second, the use of reflection

2.1 Obtaining class Objects

There are two ways to get a Class object: class.class and class.forname (). .class applies to classes that are already known at compile time.

• .class

    Class cl = String.class;
    System.out.println(cl);
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Output:

    class java.lang.String
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• Class.forName()

try { Class<? > aClass = Class.forName("java.lang.String"); System.out.println(aClass.getName()); } catch (ClassNotFoundException e) {system.out.println (" find String class "); }Copy the code

Output:

    java.lang.String
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2.2 Obtaining a class member

Student test class:

public class Student { private String name; private Integer age; public Student(String name, Integer age) { this.name = name; this.age = age; System.out.println("name: " + name + " age: " + age); } public Student(String name) { this.name = name; System.out.println("name: " + name); } public Student() { } private Student(Integer age) { this.age = age; System.out.println("age: " + age); } public String getName() { return name; } public void setName(String name) { this.name = name; } public Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; }}Copy the code

2.2.1 Getting the constructor of the class

Get all constructors:

public static void main(String[] args) throws ClassNotFoundException { Class<? > aClass = Class.forName("com.example.demo.thread.Student"); Constructor<? >[] declaredConstructors = aClass.getDeclaredConstructors(); for (Constructor constructor : DeclaredConstructors) {// Get the constructors permission modifier system.out. print(" Permission modifier: "+ Modifier. ToString (constructive.getModiFIERS ()) + "); / / get a constructor parameter of type Class [] parameterTypes = constructor. GetParameterTypes (); for (int i = 0; i < parameterTypes.length; i++) { System.out.print(parameterTypes[i].getSimpleName() + " "); } System.out.println(" "); }}Copy the code

Output:

Permission modifier: private Parameter type: Integer Permission modifier: public Parameter type: public Parameter type: String Permission modifier: public parameter type: String IntegerCopy the code

Gets a specific constructor:

Get the parameterless constructor:

try { Constructor<? > constructor = aClass.getConstructor(); System.out.println(constructor); } catch (NoSuchMethodException e) { e.printStackTrace(); }Copy the code

Output:

public com.example.demo.thread.Student()
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Get the parameterized constructor:

Class[] p = {String.class, Integer.class}; try { Constructor<? > constructor = aClass.getConstructor(p); System.out.print(Modifier. ToString (constructive.getModiFIERS ()) + "); Class[] parametertypes = constructor.getParameterTypes(); for (int j = 0; j < parametertypes.length; j++) { System.out.print(parametertypes[j].getSimpleName() + " "); } } catch (NoSuchMethodException e) { e.printStackTrace(); }Copy the code

Output:

Public parameter :String IntegerCopy the code

Call the constructor:

Class[] p = {String.class, Integer.class}; try { Constructor<? > constructor = aClass.getConstructor(p); Object instance = constructor. NewInstance (" zhang Shan ", 22); System.out.println(instance); } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) { e.printStackTrace(); }Copy the code

Output:

Name: zhang age: 22 com. Example. Demo. Thread. Student @ 4 d7e1886Copy the code

Call the private constructor:

Class[] p1 = {Integer.class}; try { Constructor<? > constructor = aClass.getDeclaredConstructor(p1); constructor.setAccessible(true); Student instance = (Student) constructor.newInstance(20); System.out.println(instance.getAge()); } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) { e.printStackTrace(); }Copy the code

Output:

20
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2.2.2 Method of obtaining a class

    try {
        Method method = aClass.getDeclaredMethod("setAge", Integer.class);
        Object arg1s[] = {10};
        Student student1 = Student.class.newInstance();
        method.invoke(student1, arg1s);
        System.out.println(student1.getAge());
    } catch (NoSuchMethodException | InvocationTargetException | InstantiationException | IllegalAccessException e) {
        e.printStackTrace();
    }
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Output:

10
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2.2.3 Obtaining member variables of a class

    try {
        Field field = aClass.getDeclaredField("age");
        Student student = new Student("zs", 21);
        try {
            field.setAccessible(true);
            field.set(student, 20);
            System.out.println(student.getAge());
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    }
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Output:

20
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