新的SimpleDateFormat始终为给定的dateFormat返回相同的引用 [英] new SimpleDateFormat always returns same reference for a given dateFormat

问题描述

我试图通过在多个线程中使用相同的SimpleDateFormat实例来复制错误.但是，我陷入了另一个问题，没有找到任何答案.

I was trying to replicate a bug by using the same instance of SimpleDateFormat across multiple threads. However I got stuck with another problem and did not find any answers to it.

这个简单的代码块复制了我所看到的问题.

This simple code block replicates the issues I am seeing.

DateFormat d1 = new SimpleDateFormat("ddMMyyyy");
DateFormat d2 = new SimpleDateFormat("ddMMyyyy");
DateFormat d3 = new SimpleDateFormat("ddMMyy");
System.out.println("d1 = " + d1);
System.out.println("d2 = " + d2);
System.out.println("d3 = " + d3);

在Java 7(1.7_0_21)下执行此操作的结果如下

The results of this operations under java 7 (1.7_0_21) is as follows

d1 = java.text.SimpleDateFormat@c5bfbc60
d2 = java.text.SimpleDateFormat@c5bfbc60
d3 = java.text.SimpleDateFormat@b049fd40

您可以看到，尽管我正在为d1和d2创建新对象，但它们最终都是相同的引用.d3由于模式不同而最终成为一个新实例.

As you can see that although I am creating new objects for d1 and d2 they end up being the same reference. d3 ends up being a new instance as pattern is different.

java编译/运行时是否进行此优化?任何指针都会有所帮助

Does java compile/runtime do this optimization? Any pointers will be helpful

推荐答案

SimpleDateFormat 或 DateFormat ( SimpleDateFormat 超类)或 Format(超类)实现了 toString()，因此 Object toString()>类实际上已执行，其代码为:

SimpleDateFormat nor DateFormat (SimpleDateFormat superclass) nor Format (DateFormat superclass) have a toString() implemented, so the toString() from the Object class is actually executed, whose code is :

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

现在，生成 SimpleDateFormat hashCode:

Now, SimpleDateFormat hashCode is generated:

public int hashCode()
{
    return pattern.hashCode();
    // just enough fields for a reasonable distribution
}

这意味着，如果您创建多个具有相同 pattern 的 SimpleDateFormat 实例，就像您的情况一样，它们将具有相同的 hashCode ，因此 toString()对于这些实例将返回相同的结果.

Which means that if you create numerous SimpleDateFormat instances with the same pattern, like in your case, they will have the same hashCode and hence toString() will return the same for these instances.

此外，正如rixmath发现的那样，具有相同 pattern 的 SimpleDateFormat 实例也将相等.

Moreover, as it has been spotted by rixmath, SimpleDateFormat instances with the same pattern will also be equal.

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