Java中的瞬态关键字及其使用

最近，我在一个朋友的一个研究项目中遇到了十字架，他们正在学习Java编程的基础知识，一些忘记的敏感信息打印在文本文件中，并记住了Java中的瞬时关键字。

Java中的瞬时关键字在安全性方面起着重要作用，并且在上述类似的“事故”中非常有用，因为它将阻止敏感信息的传输，例如密码到文件，JSON消息等需要序列化的信息。

长话短说，如果将任何变量定义为瞬态，除非将其定义为static或final ，否则它将不会序列化。

让我们来看一些例子。

在下面的示例中，我们将定义一些瞬态变量，我们将通过将它们写入文件，读取它们并查看影响来序列化它们。

import java.io.*; 
public class TestTransient implements Serializable 
{ // Normal variables String a = "JCG";String b = "IS"; // Transient variables transient String c = "GREAT"; public static void main(String[] args) throws Exception { TestTransient foo = new TestTransient(); System.out.println("a before = " + foo.a); System.out.println("b before  = " + foo.b); System.out.println("c before = " + foo.c); System.out.println("---------------------"); // serialization FileOutputStream fileOutputStream = new FileOutputStream("abc.txt"); ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream); objectOutputStream.writeObject(foo); // de-serialization FileInputStream fileInputStream = new FileInputStream("abc.txt"); ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream); TestTransient output = (TestTransient) objectInputStream.readObject(); System.out.println("a from file  = " + output.a); System.out.println("b from file  = " + output.b); System.out.println("c from file = " + output.c); } 
}

输出为：

a before = JCG
b before  = IS
c before = GREAT
-----------------------
a from file  = JCG
b from file  = IS
c from file = null

我们可以看到，标记为瞬态的变量c在序列化后丢失了其值。

让我们来看另一个例子。

import java.io.*; 
public class TestTransient implements Serializable 
{ // Normal variables String a = "JCG";String b = "IS"; // Transient variables transient static String c = "GREAT"; transient final String d = "AGAIN!"; public static void main(String[] args) throws Exception { TestTransient foo = new TestTransient(); System.out.println("a before = " + foo.a); System.out.println("b before  = " + foo.b); System.out.println("c before = " + foo.c); System.out.println("d before = " + foo.d); System.out.println("---------------------"); // serialization FileOutputStream fileOutputStream = new FileOutputStream("abc.txt"); ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream); objectOutputStream.writeObject(foo); // de-serialization FileInputStream fileInputStream = new FileInputStream("abc.txt"); ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream); TestTransient output = (TestTransient) objectInputStream.readObject(); System.out.println("a from file  = " + output.a); System.out.println("b from file  = " + output.b); System.out.println("c from file = " + output.c); System.out.println("d from file = " + output.d); } 
}

输出为：

a before = JCGb before  = ISc before = GREATd before = AGAIN!
------------------------a from file  = JCGb from file  = ISc from file = GREATd from file = AGAIN!

那么这里发生了什么？为什么同时打印c和d变量？答案是因为它们两个都被标记为静态或最终的。