What is reflection?

Mainly refers to the ability of a program to access, detect and modify its own status or behavior.

What is Java reflection?

Java reflection means that in the running state of the Java program, for any class, all properties and methods of this class can be obtained; for a given object, all properties and methods can be called Any of its properties and methods. This method of dynamically obtaining the content of a class and dynamically calling objects is called reflection mechanism.

The Java reflection mechanism mainly provides the following functions:

* 1. Determine the class to which any object belongs at runtime.

* 2. Construct an object of any class at runtime.

* 3. Determine the member variables and methods of any class at runtime.

* 4. Call the method of any object at runtime.

Java’s reflection mechanism is mainly used to analyze class capabilities. We can obtain information about members and members of each type in a program or assembly at runtime. The general object types in the program are confirmed during compilation. Java's reflection mechanism can dynamically create objects and call their properties during runtime. Therefore, the core of reflection is to dynamically load classes or call methods to access properties during runtime. It does not need to know in advance who the running object is.

Reflection can be used to:

Construct an object of any class during runtime. Determine the class to which any object belongs/whether it is an instance of a certain class during runtime. Obtain the properties and methods of any class, including inherited public methods that call methods of any object during runtime.

Example:

a) Get method

The routine DumpMethods class demonstrates the basic role of the Reflection API. It reads the class name, and then prints the method information of this class.

import java.lang.reflect.Method;
 
public class DumpMethods
{
    public static void main(String[] args) throws Exception //在方法后加上這句，異常就消失了
    {
        //獲得字符串所標(biāo)識(shí)的類的class對(duì)象
        Class<?> classType = Class.forName("java.lang.String");//在此處傳入字符串指定類名，所以參數(shù)獲取可以是一個(gè)運(yùn)行期的行為，可以用args[0]
 
        //返回class對(duì)象所對(duì)應(yīng)的類或接口中，所聲明的所有方法的數(shù)組（包括私有方法）
        Method[] methods = classType.getDeclaredMethods();
 
        //遍歷輸出所有方法聲明
        for(Method method : methods)
        {
            System.out.println(method);
        }
    }
 
}

b) Calling methods through reflection

import java.lang.reflect.Method;
 
public class InvokeTester
{
    public int add(int param1, int param2)
    {
        return param1 + param2;
 
    }
 
    public String echo(String message)
    {
        return "Hello: " + message;
    }
 
    public static void main(String[] args) throws Exception
    {
 
        // 以前的常規(guī)執(zhí)行手段
        InvokeTester tester = new InvokeTester();
        System.out.println(tester.add(1, 2));
        System.out.println(tester.echo("Tom"));
        System.out.println("---------------------------");
 
        // 通過(guò)反射的方式
 
        // 第一步，獲取Class對(duì)象
        // 前面用的方法是：Class.forName()方法獲取
        // 這里用第二種方法，類名.class
        Class<?> classType = InvokeTester.class;
 
        // 生成新的對(duì)象：用newInstance()方法
        Object invokeTester = classType.newInstance();
        System.out.println(invokeTester instanceof InvokeTester); // 輸出true
 
        // 通過(guò)反射調(diào)用方法
        // 首先需要獲得與該方法對(duì)應(yīng)的Method對(duì)象
        Method addMethod = classType.getMethod("add", new Class[] { int.class,
                int.class });
        // 第一個(gè)參數(shù)是方法名，第二個(gè)參數(shù)是這個(gè)方法所需要的參數(shù)的Class對(duì)象的數(shù)組
 
        // 調(diào)用目標(biāo)方法
        Object result = addMethod.invoke(invokeTester, new Object[] { 1, 2 });
        System.out.println(result); // 此時(shí)result是Integer類型
 
        //調(diào)用第二個(gè)方法
        Method echoMethod = classType.getDeclaredMethod("echo", new Class[]{String.class});
        Object result2 = echoMethod.invoke(invokeTester, new Object[]{"Tom"});
        System.out.println(result2);
 
    }
}

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