Java 并发数 : is final field (initialized in constructor) thread-safe?

Question

谁能告诉我这个类是否是线程安全的？

class Foo {

    private final Map<String,String> aMap;

    public Foo() {
        aMap = new HashMap<String, String>();
        aMap.put("1", "a");
        aMap.put("2", "b");
        aMap.put("3", "c");
    }

    public String get(String key) {
        return aMap.get(key);
    }

}

编辑:我没有澄清问题是我的错。根据JMM FAQ :

A new guarantee of initialization safety should be provided. If an object is properly constructed (which means that references to it do not escape during construction), then all threads which see a reference to that object will also see the values for its final fields that were set in the constructor, without the need for synchronization.

这让我感到困惑，因为 aMap 的集合是 aMap = new HashMap<String, String>(); .这样其他线程就可以看到这些

aMap.put("1", "a");
aMap.put("2", "b");
aMap.put("3", "c");

还是不是？

编辑:我找到了这个 question这完全符合我的问题

Answer 1

正如已经指出的那样，它绝对是线程安全的，final 在这里很重要，因为它具有内存可见性效果。

final 的存在保证其他线程在构造函数完成后可以在映射中看到值，而无需任何外部同步。如果没有 final，就不能在所有情况下都得到保证，并且在使新构造的对象对其他线程可用时，您需要使用安全发布习语，即(来自 Java Concurrency in Practice ) :

• Initializing an object reference from a static initializer;
• Storing a reference to it into a volatile field or AtomicReference;
• Storing a reference to it into a final field of a properly constructed object; or
• Storing a reference to it into a field that is properly guarded by a lock.