Reflection in Java

reflection

Reflection是 Java API , 用于在运行时检查或修改方法、类和接口的行为。

Reflection所需的类在java.lang.reflect这个包。
Reflection为我们提供了某对象属于哪个类，以及该类的哪些方法可以被该对象调用。
通过Reflection，我们可以在运行时调用方法(invoke methods)，而无需考虑它们private等限制。访问限定符（access specifier）

Reflection 可以用来获取以下信息

类：使用方法getClass()，获取某个对象属于哪个类。
构造函数：使用方法 getConstuctor()，获取对象所属的类的公开构造函数。
方法：使用getMethod()获取对象所属的类的公开方法。

coding

主要学习

通过field name来创建对象，这需要调用getDeclaredField()方法；

通过设置setAccessible(truee)来避开field的限制，比如private的变量，类外面的函数可以访问。

// creates object of the desired field by providing
// the name of field as argument to the
// getDeclaredField method
Field field = cls.getDeclaredField("s");  // s is private field name

// 通过setAccessible方法来避开field的限制
field.setAccessible(true);

// takes object and the new value to be assigned
// to the field as arguments
field.set(obj, "contents to override field s");

示例代码

// A simple Java program to demonstrate the use of reflection
import java.lang.reflect.Method;
import java.lang.reflect.Field;
import java.lang.reflect.Constructor;


class Test1
{
	public static void main(String args[]) throws Exception
	{
		// Creating object whose property is to be checked
        // 创建对象
		Test obj = new Test();

		// Creating class object from the object using
		// getclass method
        // 使用getClass方法创建对象
		Class cls = obj.getClass();
		System.out.println("The name of class is " +
							cls.getName());

		// Getting the constructor of the class through the
		// object of the class
        // 使用getConstructor方法来获取构造函数
		Constructor constructor = cls.getConstructor();
		System.out.println("The name of constructor is " +
							constructor.getName());

		System.out.println("The public methods of class are : ");

		// Getting methods of the class through the object
		// of the class by using getMethods
		Method[] methods = cls.getMethods();

		// Printing method names
		for (Method method:methods)
			System.out.println(method.getName());
        
        System.out.println();
	
		// creates object of desired method by providing the
		// method name and parameter class as arguments to
		// the getDeclaredMethod
		Method methodcall1 = cls.getDeclaredMethod("method2",
												int.class);

		// invokes the method at runtime
		methodcall1.invoke(obj, 19);

		// creates object of the desired field by providing
		// the name of field as argument to the
		// getDeclaredField method
		Field field = cls.getDeclaredField("s");  // private field here

		// allows the object to access the field irrespective
		// of the access specifier used with the field
        // 通过setAccessible方法来避开field的限制
		field.setAccessible(true);

		// takes object and the new value to be assigned
		// to the field as arguments
		field.set(obj, "JAVA");

		// Creates object of desired method by providing the
		// method name as argument to the getDeclaredMethod
		Method methodcall2 = cls.getDeclaredMethod("method1");

		// invokes the method at runtime
		methodcall2.invoke(obj);

		// Creates object of the desired method by providing
		// the name of method as argument to the
		// getDeclaredMethod method
		Method methodcall3 = cls.getDeclaredMethod("method3");

		// allows the object to access the method irrespective
		// of the access specifier used with the method
		methodcall3.setAccessible(true);

		// invokes the method at runtime
		methodcall3.invoke(obj);
	}
}


// class whose object is to be created
class Test
{
	// creating a private field 私有成员函数
	private String s;

	// creating a public constructor 构造函数
	public Test() { s = "Reflection in Java"; }

	// Creating a public method with no arguments
	public void method1() {
		System.out.println("The string is " + s);
	}

	// Creating a public method with int as argument
	public void method2(int n) {
		System.out.println("The number is " + n);
	}

	// creating a private method
	private void method3() {
		System.out.println("Private method invoked");
	}
}

测试结果

$ java Test1.java
Note: Test1.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
The name of class is Test
The name of constructor is Test
The public methods of class are :
method2
method1
wait
wait
wait
equals
toString
hashCode
getClass
notify
notifyAll

The number is 19
The string is JAVA
Private method invoked

summary: In Java, we can use the features of reflection to modify private fields. 今天学习了Java 的reflection。

参考：https://www.geeksforgeeks.org/reflection-in-java/