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blog-site/content/posts/java/javakeyword/transient关键字详解.md
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| --- | ||
| title: "transient关键字详解" | ||
| date: 2024-08-20 | ||
| draft: true | ||
| date: 2024-08-18 | ||
| draft: false | ||
| tags: ["Java", "关键字","详解"] | ||
| slug: "java-keyword-transient" | ||
| --- | ||
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| ## transient | ||
| transient关键字在Java中用于声明一个类的成员变量，它表示该变量不应被序列化。当对象序列化时，该字段不会被持久化到目标字节流中。也就是说，使用transient修饰的字段在序列化过程中将被忽略，反序列化时这些字段会被初始化为默认值，基本数据类型为0，引用类型为null。 | ||
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| 这对于敏感信息或不需要持久化的数据非常有用。比如说用户的一些敏感信息，如密码、银行卡号等，为了安全起见，不希望在网络操作中传输或者持久化到磁盘文件中，那这些字段就可以加上transient关键字。 | ||
| 通过避免不必要的数据序列化尤其是一些内存较大的字段，可以减少序列化和反序列化的时间、内存和存储空间消耗，提升性能。 | ||
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| ### 使用示例 | ||
| 一旦字段被transient修饰，成员变量将不再是对象持久化的一部分，该变量的值在序列化后无法访问。 | ||
| transient只能用于类的成员变量，不能用于方法、局部变量或类，这使得它的应用范围相对有限。 | ||
| ```java | ||
| public class User implements Serializable { | ||
| private static final long serialVersionUID = 1L; | ||
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| private String username; | ||
| private transient String password; | ||
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| public User(String username, String password) { | ||
| this.username = username; | ||
| this.password = password; | ||
| } | ||
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| @Override | ||
| public String toString() { | ||
| return "User{" + | ||
| "username='" + username + '\'' + | ||
| ", password='" + password + '\'' + | ||
| '}'; | ||
| } | ||
| } | ||
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| public class Main { | ||
| public static void main(String[] args) { | ||
| User user = new User("JohnDoe", "password123"); | ||
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| // 序列化 | ||
| try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user.ser"))) { | ||
| oos.writeObject(user); | ||
| } catch (IOException e) { | ||
| e.printStackTrace(); | ||
| } | ||
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| // 反序列化 | ||
| try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user.ser"))) { | ||
| User deserializedUser = (User) ois.readObject(); | ||
| // User{username='JohnDoe', password='null'} | ||
| System.out.println(deserializedUser); | ||
| } catch (IOException | ClassNotFoundException e) { | ||
| e.printStackTrace(); | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
| 被transient关键字修饰的字段不能被序列化，但是一个静态变量不管是否被transient修饰，均不能被序列化。 | ||
| 这是因为static字段属于类而不是某个具体的对象，序列化是对象级别的操作，它保存的是对象的实例状态，而static字段是类级别的状态，不属于任何具体对象，因此在序列化过程中被忽略。 | ||
| ```java | ||
| class Example implements Serializable { | ||
| private static final long serialVersionUID = 1L; | ||
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| private int instanceVar; | ||
| private static int staticVar = 100; // static字段 | ||
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| public Example(int instanceVar) { | ||
| this.instanceVar = instanceVar; | ||
| } | ||
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| @Override | ||
| public String toString() { | ||
| return "Example{" + | ||
| "instanceVar=" + instanceVar + | ||
| ", staticVar=" + staticVar + | ||
| '}'; | ||
| } | ||
| } | ||
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| public class Main { | ||
| public static void main(String[] args) { | ||
| Example example = new Example(42); | ||
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| // 序列化 | ||
| try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("example.ser"))) { | ||
| oos.writeObject(example); | ||
| } catch (IOException e) { | ||
| e.printStackTrace(); | ||
| } | ||
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| // 修改 static 字段的值 | ||
| Example.staticVar = 200; | ||
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| // 反序列化 | ||
| try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream("example.ser"))) { | ||
| Example deserializedExample = (Example) ois.readObject(); | ||
| System.out.println(deserializedExample); | ||
| } catch (IOException | ClassNotFoundException e) { | ||
| e.printStackTrace(); | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| ### transient与序列化 | ||
| 在Java中，对象的序列化可以通过实现两种接口来实现，如果实现的是`Serializable`接口，则所有的序列化将会自动进行。 | ||
| 如果实现的是`Externalizable`接口，则需要在`writeExternal`方法中指定要序列化的字段，与transient关键字修饰无关。 | ||
| ```java | ||
| class Employee implements Externalizable { | ||
| private String name; | ||
| private transient int age; // `transient` 在 `Externalizable` 中不起作用 | ||
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| public Employee() {} // 无参构造函数必须存在 | ||
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| public Employee(String name, int age) { | ||
| this.name = name; | ||
| this.age = age; | ||
| } | ||
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| @Override | ||
| public void writeExternal(ObjectOutput out) throws IOException { | ||
| out.writeObject(name); | ||
| out.writeInt(age); // 手动指定要序列化的字段 | ||
| } | ||
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| @Override | ||
| public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { | ||
| name = (String) in.readObject(); | ||
| age = in.readInt(); // 手动指定如何反序列化 | ||
| } | ||
| } | ||
| ``` | ||
| `Serializable`是Java标准库提供的接口，而`Externalizable`是`Serializable`的子接口。 | ||
| 使用`Externalizable`接口需要实现`readExternal`和`writeExternal`方法，需要手动完成序列化和反序列化的过程。 | ||
| 因为需要手动完成，所以`Externalizable`接口提供了更高的序列化控制能力，可以在序列化和反序列化过程中对对象进行自定义的处理，如对一些敏感信息进行加密和解密。 | ||
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| ### transient底层原理 | ||
| transient译为“临时的”，它可以阻止字段被序列化到文件中，在被反序列化后，transient字段的值被设为初始值，比如int型的初始值为0，对象型的初始值为null。 | ||
| static和transient修饰的字段是不会被序列化的，在序列化底层代码中可以看到相关源代码`ObjectStreamClass`方法： | ||
| ```java | ||
| private static ObjectStreamField[] getDefaultSerialFields(Class<?> cl) { | ||
| // 获取该类中声明的所有字段 | ||
| Field[] clFields = cl.getDeclaredFields(); | ||
| ArrayList<ObjectStreamField> list = new ArrayList<>(); | ||
| int mask = Modifier.STATIC | Modifier.TRANSIENT; | ||
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| // 遍历所有字段，将非 static 和 transient 的字段添加到 list 中 | ||
| for (int i = 0; i < clFields.length; i++) { | ||
| Field field = clFields[i]; | ||
| int mods = field.getModifiers(); | ||
| if ((mods & mask) == 0) { | ||
| // 根据字段名、字段类型和字段是否可序列化创建一个 ObjectStreamField 对象 | ||
| ObjectStreamField osf = new ObjectStreamField(field.getName(), field.getType(), !Serializable.class.isAssignableFrom(cl)); | ||
| list.add(osf); | ||
| } | ||
| } | ||
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| int size = list.size(); | ||
| // 如果 list 为空，则返回一个空的 ObjectStreamField 数组，否则将 list 转换为 ObjectStreamField 数组并返回 | ||
| return (size == 0) ? NO_FIELDS : | ||
| list.toArray(new ObjectStreamField[size]); | ||
| } | ||
| ``` | ||
| 当一个类中的字段被标记为 transient，Java的序列化机制在序列化对象时，会忽略这些transient字段。具体来说，序列化过程中会跳过这些字段，不将其写入到对象的字节流中。 | ||
| ```java | ||
| int mask = Modifier.STATIC | Modifier.TRANSIENT; | ||
| ``` | ||
| 这段代码表明，这两个修饰符标记的字段就没有被放入到序列化的字段中。 |
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