Java中的内存泄露

Q：在Java中怎么可以产生内存泄露？

A：Java中，造成内存泄露的原因有很多种。典型的例子是一个没有实现hasCode和

equals方法的Key类在HashMap中保存的情况。最后会生成很多重复的对象。所有的内存泄露

最后都会抛出OutOfMemoryError异常，下面通过一段简短的通过无限循环模拟内存泄露

的例子说明一下。

 

 

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);
 }
 
}

 

 

结果如下：

 

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:怎么解决上面的内存泄露？

A：实现Key类的equals和hasCode方法。

 

   .....
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();
 }
}
.....

 

重新执行程序会得到如下结果：

 

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：在实际场景中，你怎么查找内存泄露？

A：通过以下代码获取线程ID

 

C:\>jps
5808 Jps
4568 MemoryLeak
3860 Main

 

通过命令行打开jconsole

 

C:\>jconsole 4568

 

实现了hasCode和equals的Key类和没有实现的图表如下所示：

 

没有内存泄露的：

 

 

造成内存泄露的：