Q: How can memory leak occur in Java?
A: In Java, there are many reasons for memory leaks. A typical example is a code that does not implement hasCode and
The Key class of equals method is saved in HashMap. In the end, many duplicate objects will be generated. All memory leaks
In the end, an OutOfMemoryError exception will be thrown. Here is a brief simulation of a memory leak through an infinite loop.
Explain with an example.
Copy the code code as follows:
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 result is as follows:
Copy the code code 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.
Copy the code code as follows:
.....
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 yield the following results:
Copy the code code as follows:
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
Copy the code code as follows:
C:/>jps
5808Jps
4568 MemoryLeak
3860 Main
Open jconsole via command line
Copy the code code as follows:
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: