memory java heap

memory java heap

java可以通过java.nio.ByteBuffer.allocateDirect(capacity)直接使用non java heap(java堆外)的内存 。

一.使用目的：

1。开辟数据缓冲区

2。可以突破jvm内存限制，操作更多的物理内存(不同的jvm实现不一样,sun jvm会被限制，但能增加jvm能够操纵的一倍内存,而jrockit基本上可以将物理内存全部耗光)

二.使用问题:

通过ByteBuffer.allocateDirect()使用的内存不能够通过jvm相关内存工具:VisualVM等查看heap的内存占用,所以内存泄露也较难查找。

三. 测试如下:

public class ByteBufferTest {

public static void main(String[] args) throws InterruptedException {

System.out.MemoryInfo());

List list = new ArrayList();

for(int i = 0; i < 100; i++) {

list.add(ByteBuffer.allocateDirect(Integer.MAX_VALUE / 100));

System.out.println(i + " " + MemoryInfo()); // 自己写的内存占用工具类

}

System.out.println("start sleep,list.size():"+list.size());

Thread.sleep(100000);

System.out.println(list.size());

// java.nio.ByteBuffer.allocate(capacity) 使用 java heap(java堆)分配内存: 可以直接通过工具查看出内存占用

// java.nio.ByteBuffer.allocateDirect(capacity) 使用non java heap(非java堆)通过操作系统直接分配内存: 会导致查看不出java heap的内存占用，导致泄露看不出来

}

}

打印结果:

----------------------

totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1285MB usedPhysicalMemory:762MB

0 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1264MB usedPhysicalMemory:783MB

1 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1244MB usedPhysicalMemory:803MB

2 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1223MB usedPhysicalMemory:824MB

3 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1203MB usedPhysicalMemory:844MB

4 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1182MB usedPhysicalMemory:865MB

5 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1162MB usedPhysicalMemory:885MB

6 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1141MB usedPhysicalMemory:906MB

7 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1121MB usedPhysicalMemory:926MB

8 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1100MB usedPhysicalMemory:947MB

9 totalHeapMemory:15MB freeHeapMemory:14MB maxHeapMemory:247MB totalPhysicalMemorySize:2047MB freePhysicalMemorySize:1079MB usedPhysicalMemory:968MB

VisualVM内存查看:

Heap内存分配

实际java进程内存占用:

通过VisualVM可以发现，java heap根本没有使用，但java进程的物理内存已经使用400MB+

四.测试结论:

1. 使用ByteBuffer.allocateDirect()分配出去的内存无法通过Heap查看

2. ByteBuffer.allocateDirect()分配的内容可以被GC回收,但泄露的话较难查找

大小: 21 KB

大小: 40.2 KB

大小: 10.2 KB

1

顶

1

踩

分享到：

2012-04-26 13:09

浏览 14352

评论