Java中典型的內存泄露問題和解決方法

WindySky 2016-05-11

展開全文

Q：在Java中怎么可以產生內存泄露？
A：Java中，造成內存泄露的原因有很多種。典型的例子是一個沒有實現(xiàn)hasCode和
equals方法的Key類在HashMap中保存的情況。最后會生成很多重復的對象。所有的內存泄露
最后都會拋出OutOfMemoryError異常，下面通過一段簡短的通過無限循環(huán)模擬內存泄露
的例子說明一下。

復制代碼代碼如下:

import java.util.HashMap;
import java.util.Map;

public class MemoryLeak {

public static void main(String[] args) {
Map<Key, String> map = new HashMap<Key, String>(1000);

int counter = 0;
while (true) {
       // creates duplicate objects due to bad Key class
   map.put(new Key("dummyKey"), "value");
   counter++;
   if (counter % 1000 == 0) {
    System.out.println("map size: " + map.size());
    System.out.println("Free memory after count " + counter
      + " is " + getFreeMemory() + "MB");

    sleep(1000);
   }


}
}

// inner class key without hashcode() or equals() -- bad implementation
static class Key {
private String key;

public Key(String key) {
   this.key = key;
}

}

//delay for a given period in milli seconds
public static void sleep(long sleepFor) {
try {
   Thread.sleep(sleepFor);
} catch (InterruptedException e) {
   e.printStackTrace();
}
}

//get available memory in MB
public static long getFreeMemory() {
return Runtime.getRuntime().freeMemory() / (1024 * 1024);
}

}

結果如下：

復制代碼代碼如下:

map size: 1000
Free memory after count 1000 is 4MB
map size: 2000
Free memory after count 2000 is 4MB
map size: 1396000
Free memory after count 1396000 is 2MB
map size: 1397000
Free memory after count 1397000 is 2MB
map size: 1398000
Free memory after count 1398000 is 2MB
map size: 1399000
Free memory after count 1399000 is 1MB
map size: 1400000
Free memory after count 1400000 is 1MB
map size: 1401000
Free memory after count 1401000 is 1MB
.....
.....
map size: 1452000
Free memory after count 1452000 is 0MB
map size: 1453000
Free memory after count 1453000 is 0MB
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.HashMap.addEntry(HashMap.java:753)
at java.util.HashMap.put(HashMap.java:385)
at MemoryLeak.main(MemoryLeak.java:10)

Q:怎么解決上面的內存泄露？
A：實現(xiàn)Key類的equals和hasCode方法。

復制代碼代碼如下:

    .....
static class Key {
private String key;

public Key(String key) {
this.key = key;
}

@Override
public boolean equals(Object obj) {

if (obj instanceof Key)
   return key.equals(((Key) obj).key);
else
   return false;

}

@Override
public int hashCode() {
return key.hashCode();
}
}
.....

重新執(zhí)行程序會得到如下結果：

復制代碼代碼如下:

map size: 1
Free memory after count 1000 is 4MB
map size: 1
Free memory after count 2000 is 4MB
map size: 1
Free memory after count 3000 is 4MB
map size: 1
Free memory after count 4000 is 4MB
...
Free memory after count 73000 is 4MB
map size: 1
Free memory after count 74000 is 4MB
map size: 1
Free memory after count 75000 is 4MB

Q：在實際場景中，你怎么查找內存泄露？
A：通過以下代碼獲取線程ID

復制代碼代碼如下:

C:\>jps
5808 Jps
4568 MemoryLeak
3860 Main

通過命令行打開jconsole

復制代碼代碼如下:

C:\>jconsole 4568

實現(xiàn)了hasCode和equals的Key類和沒有實現(xiàn)的圖表如下所示：

沒有內存泄露的：