可怕的表现&Java 8 构造函数引用的大堆内存占用? [英] Horrendous performance & large heap footprint of Java 8 constructor reference?

问题描述

我刚刚在我们的生产环境中遇到了相当不愉快的经历，导致 OutOfMemoryErrors: heapspace..

I just had a rather unpleasant experience in our production environment, causing OutOfMemoryErrors: heapspace..

我将问题追溯到我在函数中使用 ArrayList::new.

I traced the issue to my use of ArrayList::new in a function.

为了验证这实际上比通过声明的构造函数(t -> new ArrayList<>())创建的性能更差，我编写了以下小方法:

To verify that this is actually performing worse than normal creation via a declared constructor (t -> new ArrayList<>()), I wrote the following small method:

public class TestMain {
  public static void main(String[] args) {
    boolean newMethod = false;
    Map<Integer,List<Integer>> map = new HashMap<>();
    int index = 0;

    while(true){
      if (newMethod) {
        map.computeIfAbsent(index, ArrayList::new).add(index);
     } else {
        map.computeIfAbsent(index, i->new ArrayList<>()).add(index);
      }
      if (index++ % 100 == 0) {
        System.out.println("Reached index "+index);
      }
    }
  }
}

使用 newMethod=true; 运行该方法将导致该方法在索引达到 30k 后以 OutOfMemoryError 失败.使用 newMethod=false; 程序不会失败，但会一直冲击直到被杀死(索引很容易达到 150 万).

Running the method with newMethod=true; will cause the method to fail with OutOfMemoryError just after index hits 30k. With newMethod=false; the program does not fail, but keeps pounding away until killed (index easily reaches 1.5 milion).

为什么 ArrayList::new 在堆上创建如此多的 Object[] 元素，导致 OutOfMemoryError 如此之快?

Why does ArrayList::new create so many Object[] elements on the heap that it causes OutOfMemoryError so fast?

(顺便说一句 - 当集合类型为 HashSet 时也会发生这种情况.)

(By the way - it also happens when the collection type is HashSet.)

推荐答案

在第一种情况 (ArrayList::new) 中，您使用的是 constructor 它接受一个初始容量参数，在第二种情况下你不是.较大的初始容量(代码中的index)会导致分配较大的Object[]，从而导致OutOfMemoryErrors.

In the first case (ArrayList::new) you are using the constructor which takes an initial capacity argument, in the second case you are not. A large initial capacity (index in your code) causes a large Object[] to be allocated, resulting in your OutOfMemoryErrors.

以下是两个构造函数的当前实现:

Here are the two constructors' current implementations:

public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

HashSet 中也发生了类似的事情，只是在调用 add 之前不会分配数组.

Something similar happens in HashSet, except the array is not allocated until add is called.

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