Typical memory leak problems and solutions in Java

Q: How can memory leak occur in Java?
A: In Java, there are many reasons for memory leaks. A typical example is a Key class that does not implement the hasCode and
equals methods and is saved in a HashMap. In the end, many duplicate objects will be generated. All memory leaks
will eventually throw an OutOfMemoryError exception. The following is a brief example of simulating a memory leak through an infinite loop
.

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);
 }

}

The results are as follows:

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: How to solve the above memory leak?
A: Implement the equals and hasCode methods of the Key class.

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

Re-executing the program will get the following results:

 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: In actual scenarios, how do you find memory leaks?
A: Get the thread ID through the following code

C:\>jps
5808 Jps
4568 MemoryLeak
3860 Main

Open jconsole through the command line

C:\>jconsole 4568

The Key class that implements hasCode and equals and the chart that does not implement it are as follows:

No memory leak:

Cause memory leak:

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