How can I create a memory leak in Java?

Michael N.

—January 30, 2023

The problem

Java is famous for its garbage collector (GC), which automatically clears unused objects. This means programmers no longer need to manually handle memory management as they do in languages like C and C++. Unfortunately, in some cases, the GC cannot clear objects from memory that are no longer in use and this causes memory leaks. This leads to objects building up in memory and possibly causing an OutofMemoryError exception.

You may want to create memory leaks when performance testing or profiling software, but understanding how memory leaks are created can also help us to prevent them.

The solution

There are many ways to create memory leaks. We’ll take a look at four.

Unclosed resources

A frequent cause of memory leaks is when resources such as database connections and file streams are left unclosed. Here’s an example:

private static void readFile() throws Exception {
    FileReader fileReader = new FileReader("src/file.txt");
    int character;
    while ((character = fileReader.read()) != -1) {
        System.out.print((char) character);
    }
}

Here, we’ve created and used a FileReader without calling the close() method when it’s done. This means the FileReader remains open and in memory while the application is running.

Poorly implemented hashCode and equals methods

Memory leaks are created when equals and hashCode methods on objects are either poorly implemented or not implemented. These two methods are the basis of comparing two objects in Java. When they are not properly implemented, duplicate objects are created and allocated memory by the GC. For example, HashMap and HashSet use these methods to ensure there are no repeat keys.

Not implementing these methods creates duplicate keys causing memory leaks. This is shown below:

// Student.java
public class Student {
    String name;

    Student(String name) {
        this.name = name;
    }
}

// run this method from the main application method
private static void createMap() {
    Map<Student, Integer> map = new HashMap<>();
    for (int i = 0; i < 100; i++) {
        map.put(new Student("jon"), 1);
    }
    System.out.println("Map size: " + map.size());
}

Since Map doesn’t allow duplicate keys, running this code should give us an output of Map size: 1 but instead it returns Map size: 100. This is because the Student class doesn’t implement the equals or hashCode methods, so the HashMap can’t tell that the objects created are the same.

Overriding the finalize method

The finalize method exists on all Java objects and is called when the JVM decides that an object should be garbage collected. You can override this method to create a function that doesn’t resolve or runs for a long time, creating a memory leak. The memory leak is because once the finalize method is called, the GC has to wait for the method to resolve (that is, finalize) before removing it from the heap.

Static fields

Another common cause of memory leaks is static fields. Static fields are kept in memory for the entire lifespan of the application, which is especially problematic for static fields that hold large objects. Too many static objects kept in memory will cause an OutofMemoryError exception as the GC won’t have any space to allocate new objects.