java使用java heap外(堆外)内存导致的内存泄露
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.println(SystemRuntimeInfo.getMemoryInfo());
List list = new ArrayList();
for(int i = 0; i < 100; i++) {
list.add(ByteBuffer.allocateDirect(Integer.MAX_VALUE / 100));
System.out.println(i + " " + SystemRuntimeInfo.getMemoryInfo()); // 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