关于Java对象占用内存大小

2012-12-19

附加：以下内容很多错误，Java对象中还有很多其他内容， 比如指向类的指针，Flags（哈希值等），形状（是否是数组），如果是数组还有数组长度。后面才是正文内容。并且如果是String对象的话，String对象的正文部分是一个指向char[]的指针，这个char[]数组本身也有上面说的类型，Flags，形状，数组大小等基本信息和实际内容。并且在32bit和64bit的JVM上Java对象的基本结构占用内存也是不同的

昨天面试有一道题是关于Java内存占用的。题目是求出new String("a")和Map map = new HashMap(); map.put("a", "a");

回来之后我检测了一下，通过观察创建对象前后（创建大量一样的对象求均值）的已使用堆（runtime.totalMemory() - runtime.freeMemory())内存大小的变化来计算Java对象的内存占用情况。

代码是网上找来的：

import java.util.HashMap;
import java.util.Map;
public class Test5 {
    public static void main(String[] args) throws Exception {
        // Warm up all classes/methods we will use
        runGC();
        usedMemory();
        // Array to keep strong references to allocated objects
        final int count = 100000;
        Object[] objects = new Object[count];
        long heap1 = 0;
        // Allocate count+1 objects, discard the first one
        for (int i = -1; i < count; ++i) {
            Object object = null;
            // Instantiate your data here and assign it to object
            // object = new Object();
            // object = new Integer (i);
            // object = new Long (i);
            // object = new String ("a");
            Map object1 = new HashMap();
            object1.put("a", "a");
            // object1.put(new String("a"), new String("a"));
            // object = new byte [128][1]
            if (i >= 0)
                objects[i] = object1;
            else {
                object1 = null; // Discard the warm up object
                runGC();
                heap1 = usedMemory(); // Take a before heap snapshot
            }
        }
        runGC();
        long heap2 = usedMemory(); // Take an after heap snapshot:
        final int size = Math.round(((float) (heap2 - heap1)) / count);
        System.out.println("'before' heap: " + heap1 + ", 'after' heap: "
                            + heap2);
        System.out.println("heap delta: " + (heap2 - heap1) + ", {"
                            + objects[0].getClass() + "} size = " + size + " bytes");
        for (int i = 0; i < count; ++i)
            objects[i] = null;
            objects = null;
        }

    private static void runGC() throws Exception {
        // It helps to call Runtime.gc()
        // using several method calls:
        for (int r = 0; r < 4; ++r)
            _runGC();
    }

    private static void _runGC() throws Exception {
        long usedMem1 = usedMemory(), usedMem2 = Long.MAX_VALUE;
        for (int i = 0; (usedMem1 < usedMem2) && (i < 500); ++i) {
            s_runtime.runFinalization();
            s_runtime.gc();
            Thread.currentThread().yield();
            usedMem2 = usedMem1;
            usedMem1 = usedMemory();
        }
    }

    private static long usedMemory() {
        return s_runtime.totalMemory() - s_runtime.freeMemory();
    }

    private static final Runtime s_runtime = Runtime.getRuntime();
}

运行结果表明：

new String(“a”)占用24Bytes,
Map map = new HashMap(); map.put(“a”, “a”); 占用168Bytes,
Map map = new HashMap(); map.put(new String(“a”), new String(“a”));占用216Bytes.

原理：Java 的String 对象有四个属性：

privatefinalcharvalue[];
/** The offset is the first index of the storage that is used. */
privatefinalintoffset;
/** The count is the number of characters in the String. */
privatefinalintcount;
/** Cache the hash code for the string */
private inthash;// Default to 0

再考虑Java对象的8字节对齐，java采用utf16编码，每个char 占用2个字节，另外，java对象有两个字（每个字2字节）的头部，分别表示垃圾收集、hash码信息以及指向对象的类，即占有4个字节。 24字节应该还是差不多的。

不过网上还有种说法是一个空字符串都占用40个字节，长为1的字符串也是占用40个字节，不清楚怎么算的，昨天面试官说的也是大概这个数据。

引用：http://www.javaworld.com/javaworld/javatips/jw-javatip130.html?page=1