Java中典型的内存泄露问题和解决方法

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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。

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: