Java中ArrayList的内存大小是多少

Question

I have an ArrayList<Obj> and I wish to know how much memory it is using.

The Obj is variant so, it is not as easy as multiply the number of elements in the array per the size of an object.

Answer 1

It's the capacity of the java.util.ArrayList multiplied by the reference size (4 bytes on 32bit, 8bytes on 64bit) + [Object header + one int and one references] .

The capacity is different (always >=) than the size but you want to make 'em equal, call trimToSize()

Technically speaking the ArrayList has an Object[] where it stores the data.

Answer 2

您可以使用类似Runtime.getRuntime().totalMemory()及其对应的Runtime.getRuntime().freeMemory()来进行有根据的猜测，但这并不考虑两次调用之间使用GC的对象。

Answer 3

This is what a memory profiler is for. It will tell you for your platform. The minimum size for an empty ArrayList is 64-bytes. It is highly likely you don't need to know this unless you have 100K elements or more.

Answer 4

You can brute force calculate it if you know the content distribution of the objects.

However, the most precise method I can think of is to look at it in a profiler. There are free tools out there, some that come with the JDK. However, the best tool I've used is Yourkit. That doesn't mean it's the only solution, just my favorite.

Answer 5

您可能必须使用类似于Netbeans中可用的探查器 ，该探查器将向您显示我们程序的内存消耗，并可以为您提供有关每个对象的一些详细信息。

Answer 6

The answer is, it depends on how you measure. If you asked me the size of an ArrayList I would give you the shallow size. That is to say, an ArrayList consists of an array of references and an integer indicating the number of contained elements. If you wanted to know the size it is quite simply 4 + *array.length.

If you want to know the size of the ArrayList and all contained elements then there are some heap analyzers that can figure this out. I believe YourKit is one of them.

Answer 7

The memory usage of java depends of the JVM implementation. The only real method to determine the memory usage of an instance I know is to use an Java 5 instrumentation agent. There is a little tutorial to do so.

Answer 8

The way an ArrayList works is that it simply contains an array of a certain size (which can be specified in the constructor, I believe 10 is the default?). Whenever the number of elements becomes too large for the internal array, the size of the internal array is doubled. So you need to multiply the size of the object by the size of the internal array.

Answer 9

Source: http://uttesh.blogspot.com/2015/04/get-byte-or-memory-size-of.html

public static long getBytesFromList(List list) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ObjectOutputStream out = new ObjectOutputStream(baos);
    out.writeObject(list);
    out.close();
    return baos.toByteArray().length;
}