必须是不可变对象的所有属性都是最终的吗？ [英] Must all properties of an immutable object be final?

问题描述

必须不可变对象的所有属性都是 final ？

Must immutable objects have all properties be final?

据我所知。但我不知道，我是否是对的。

According to me not. But I don't know, whether I am right.

推荐答案

不可变对象（所有属性final）和一个有效的不可变对象（属性不是最终的但不能更改）是安全的发布。

The main difference between an immutable object (all properties final) and an effectively immutable object (properties aren't final but can't be changed) is safe publication.

您可以安全地在多线程上下文中发布不可变对象而无需由于提供的保证，担心添加同步最终字段的Java内存模型：

You can safely publish an immutable object in a multi threaded context without having to worry about adding synchronization, thanks to the guarantees provided by the Java Memory Model for final fields:

最终字段还允许程序员在没有同步的情况下实现线程安全的不可变对象。线程安全的不可变对象被所有线程视为不可变对象，即使使用数据争用传递线程之间的不可变对象的引用也是如此。这可以提供安全保证，防止错误或恶意代码滥用不可变类。必须正确使用最终字段以提供不可变性的保证。

final fields also allow programmers to implement thread-safe immutable objects without synchronization. A thread-safe immutable object is seen as immutable by all threads, even if a data race is used to pass references to the immutable object between threads. This can provide safety guarantees against misuse of an immutable class by incorrect or malicious code. final fields must be used correctly to provide a guarantee of immutability.

作为旁注，它还可以强制实现不变性（如果你试图在类的未来版本中改变这些字段，因为你忘了它应该是不可变的，它不会编译）。

As a side note, it also enables to enforce immutability (if you try to mutate those fields in a future version of your class because you have forgotten it should be immutable, it won't compile).

澄清

• 使一个对象的所有字段最终都不会使它成为不可变的 - 你还需要确保（i）它的状态不能更改（例如，如果对象包含最终列表，则不得在构建后进行变异操作（添加，删除...）和（ii）您不要


• 一个有效不可变的对象一旦安全发布就是线程安全的
$ b $让在构造期间转义 b

• 不安全发布的示例：

• Making all the fields of an object final does not make it immutable - you also need to make sure that (i) its state can't change (for example, if the object contains a final List, no mutating operations (add, remove...) must be done after construction) and (ii) you don't let this escape during construction
• An effectively immutable object is thread safe once it has been safely published

• Example of unsafe publication:

class EffectivelyImmutable {
    static EffectivelyImmutable unsafe;
    private int i;
    public EffectivelyImmutable (int i) { this.i = i; }
    public int get() { return i; }
}

// in some thread
EffectivelyImmutable.unsafe = new EffectivelyImmutable(1);

//in some other thread
if (EffectivelyImmutable.unsafe != null
    && EffectivelyImmutable.unsafe.get() != 1)
    System.out.println("What???");

这个程序理论上可以打印什么??? 。如果 i 是最终的，那将不是合法的结果。

This program could in theory print What???. If i were final, that would not be a legal outcome.

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