Java中典型的内存泄露问题和解决方法

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。

复制代码代码如下:

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);

 }

}

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。

 

复制代码代码如下:

    .....

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();

 }

}

.....

 

 重新执行程序会得到如下结果：

 

复制代码代码如下:

 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

复制代码代码如下:

C:\>jps

5808 Jps

4568 MemoryLeak

3860 Main

通过命令行打开jconsole

复制代码代码如下:

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：