Java中典型的內存泄露問題和解決方法

2019-11-26 15:31:29

字體：大中小

來源：轉載

供稿：網友

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: