Java中有一个HashMap,我们也都知道,改类型为线程不安全,会出现线程死循环问题,今天我们通过一个小代码来模拟一下HashMap产生死循环,同时描述一下现象:
代码:
java代码:
package com.wmmad.threadlocal.test;import java.util.HashMap; /** * * @author madding.lip * * @date 2010.08.25 * * <pre> * 模拟HashMap循环现象 * </pre> */ public class HashMapInfiniteLoop { private HashMap hash = new HashMap(); public HashMapInfiniteLoop() { Thread t1 = new Thread() { public void run() { for (int i = 0; i < 50000; i++) { hash.put(new Integer(i), Integer.valueOf(i)); } System.out.println("t1 over"); } }; Thread t2 = new Thread() { public void run() { for (int i = 0; i < 50000; i++) { hash.put(new Integer(i),Integer.valueOf(i)); } System.out.println("t2 over"); } }; t1.start(); t2.start(); } public static void main(String[] args) { new HashMapInfiniteLoop(); } }
shell代码:
#!/bin/bash # author: madding.lip # date: 2010.08.25 # function: mutli call for i in `seq 1 1000`; do java com/wmmad/threadlocal/test/HashMapInfiniteLoop done exit 0
用shell脚本去跑,无非是增加实例数。多次操作后,就能查找偶然性中的必然性
编译java代码,再运行shell脚本,大概在运行到50左右的时候就会挂起,这是使用jps命令查看一下进程发现会有如下java进程:
29369 Jps
27424
29355 HashMapInfiniteLoop
7745
15518
10268
用jconsole链接对应的进程,查看该进程暂用资源情况
现象:
1.cpu不断飙升,直到把所有cpu资源全部占满(最典型现象)
2.Heap内存由规律折回
3.Stack trace:
java.util.HashMap.transfer(HashMap.java:484)
java.util.HashMap.resize(HashMap.java:463)
java.util.HashMap.addEntry(HashMap.java:755)
java.util.HashMap.put(HashMap.java:385)
com.wmmad.threadlocal.test.HashMapLock$2.run(HashMapLock.java:24)
可以看到挂起的代码是这里:
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
src[j] = null;
do {
Entry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}