文章目錄

• Reflection in Java
• • reflection
  • coding

Reflection in Java

reflection

Reflection是 Java API , 用于在運行時檢查或修改方法、類和接口的行為。

• Reflection所需的類在java.lang.reflect這個包。
• Reflection為我們提供了某對象屬于哪個類，以及該類的哪些方法可以被該對象調(diào)用。
• 通過Reflection，我們可以在運行時調(diào)用方法(invoke methods)，而無需考慮它們private等限制。訪問限定符（access specifier）

Reflection is an APIwhich is used to examine or modify the behavior of methods, classes, interfaces at runtime.

• The required classes for reflection are provided under java.lang.reflect package.
• Reflection gives us information about the class to which an object belongs and also the methods of that class which can be executed by using the object.
• Through reflection we can invoke methods at runtime irrespective of the access specifier used with them.

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

Reflection 可以用來獲取以下信息

• 類：使用方法getClass()，獲取某個對象屬于哪個類。
• 構(gòu)造函數(shù) ：使用方法 getConstuctor()，獲取對象所屬的類的公開構(gòu)造函數(shù)。
• 方法 ：使用getMethod()獲取對象所屬的類的公開方法。

Reflection can be used to get information about –

Class The getClass() method is used to get the name of the class to which an object belongs.

Constructors The getConstructors() method is used to get the public constructors of the class to which an object belongs.

Methods The getMethods() method is used to get the public methods of the class to which an objects belongs.

coding

主要學(xué)習(xí)

通過field name來創(chuàng)建對象，這需要調(diào)用getDeclaredField()方法；

通過設(shè)置setAccessible(truee)來避開field的限制，比如private的變量，類外面的函數(shù)可以訪問。

// 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// 通過setAccessible方法來避開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");

示例代碼

// 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// 創(chuàng)建對象Test obj = new Test();// Creating class object from the object using// getclass method// 使用getClass方法創(chuàng)建對象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方法來獲取構(gòu)造函數(shù)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 getMethodsMethod[] methods = cls.getMethods();// Printing method namesfor (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 getDeclaredMethodMethod methodcall1 = cls.getDeclaredMethod("method2",int.class);// invokes the method at runtimemethodcall1.invoke(obj, 19);// creates object of the desired field by providing// the name of field as argument to the// getDeclaredField methodField field = cls.getDeclaredField("s"); // private field here// allows the object to access the field irrespective// of the access specifier used with the field// 通過setAccessible方法來避開field的限制field.setAccessible(true);// takes object and the new value to be assigned// to the field as argumentsfield.set(obj, "JAVA");// Creates object of desired method by providing the// method name as argument to the getDeclaredMethodMethod methodcall2 = cls.getDeclaredMethod("method1");// invokes the method at runtimemethodcall2.invoke(obj);// Creates object of the desired method by providing// the name of method as argument to the// getDeclaredMethod methodMethod methodcall3 = cls.getDeclaredMethod("method3");// allows the object to access the method irrespective// of the access specifier used with the methodmethodcall3.setAccessible(true);// invokes the method at runtimemethodcall3.invoke(obj);} }// class whose object is to be created class Test {// creating a private field 私有成員函數(shù)private String s;// creating a public constructor 構(gòu)造函數(shù)public Test() { s = "Reflection in Java"; }// Creating a public method with no argumentspublic void method1() {System.out.println("The string is " + s);}// Creating a public method with int as argumentpublic void method2(int n) {System.out.println("The number is " + n);}// creating a private methodprivate void method3() {System.out.println("Private method invoked");} }

測試結(jié)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 notifyAllThe number is 19 The string is JAVA Private method invoked

summary: In Java, we can use the features of reflection to modify private fields. 今天學(xué)習(xí)了Java 的reflection。

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

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