什么是瞬态和挥发性改性剂？ [英] What are Transient and Volatile Modifiers?

问题描述

有人可以解释一下 transient 和 volatile 修饰符在Java中意味着什么？

Can someone explain what the transient and volatile modifiers mean in Java?

推荐答案

volatile 和瞬态修饰符可以是适用于类 ¹ 的字段，与字段类型无关。除此之外，它们是无关的。

The volatile and transient modifiers can be applied to fields of classes¹ irrespective of field type. Apart from that, they are unrelated.

transient 修饰符告诉Java对象序列化子系统在排除字段时序列化类的实例。然后反序列化对象时，该字段将初始化为默认值;即 null 表示引用类型，零或 false 表示基本类型。请注意JLS（参见 8.3.1.3 ）没有说 transient 意味着什么，但是按照 Java对象序列化规范。其他序列化机制可能注意字段的 transient -ness。或者他们可能会忽略它。

The transient modifier tells the Java object serialization subsystem to exclude the field when serializing an instance of the class. When the object is then deserialized, the field will be initialized to the default value; i.e. null for a reference type, and zero or false for a primitive type. Note that the JLS (see 8.3.1.3) does not say what transient means, but defers to the Java Object Serialization Specification. Other serialization mechanisms may pay attention to a field's transient-ness. Or they may ignore it.

（请注意，JLS允许将 static 字段声明为 transient 。这种组合对Java对象序列化没有意义，因为它无论如何都不会序列化静态。但是，它在其他情况下可能有意义，因此有一些理由不完全禁止它。）

(Note that the JLS permits a static field to be declared as transient. This combination doesn't make sense for Java Object Serialization, since it doesn't serialize statics anyway. However, it could make sense in other contexts, so there is some justification for not forbidding it outright.)

volatile 修饰符告诉JVM写入字段应始终同步刷新到内存，并且该字段应始终从内存中读取。这意味着可以在多线程应用程序中安全地访问和更新标记为volatile的字段，而无需使用本机或基于库的同步。同样，对volatile字段的读写也是原子的。 （这不适用于>>非易失性<<< long 或 double 字段，这可能受制于在一些JVM上撕裂。）JLS的相关部分是 8.3.1.4 ， 17.4 和 17.7 。

The volatile modifier tells the JVM that writes to the field should always be synchronously flushed to memory, and that reads of the field should always read from memory. This means that fields marked as volatile can be safely accessed and updated in a multi-thread application without using native or standard library-based synchronization. Similarly, reads and writes to volatile fields are atomic. (This does not apply to >>non-volatile<< long or double fields, which may be subject to "word tearing" on some JVMs.) The relevant parts of the JLS are 8.3.1.4, 17.4 and 17.7.

^{1 - 但不是局部变量或参数。}

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