java内存泄漏的经典案例

这篇文章主要介绍了Java中典型的内存泄露问题和解决方法，典型的内存泄露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，可以通过实现Key类的equals和hasCode方法解决这种内存泄漏问题,需要的朋友可以参考下。

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：
 复制代码 代码如下:

 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：