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Understanding Memory Leaks in Java

Joydip Kanjilal
Published: June 25, 2022

Java Developer Tutorials

When creating applications in Java, developers can create managed objects in their software using the new keyword. Programmers do not need to remove these managed objects explicitly in their code, since the garbage collector takes care of the removal of objects that are no longer required. So long as the objects are disposed of by the garbage collector, this is fine. However, if the garbage collector is unable to remove objects that are no longer referenced, there are chances of memory leaks in your applications.

A memory leak in Java refers to a state when an object no longer needed by the application remains alive in the Java Virtual Machine (JVM). Memory leaks occur when an application accidentally hangs on to object references that are no longer necessary. Memory leaks lead to performance degradation over time because of your application’s increased (and unexpected) use of memory.

This Java programming tutorial talks about memory leaks in Java, why they occur, and how developers can prevent them.

As an aside, programmers can use Application Performance Monitoring (AMP) tools and software to detect memory leaks and help track down issues causing a drain on resources. We have a great article looking at the Top Application Performance Monitoring Tools for Developers.

What is a Memory Leak?

A memory leak is a situation in which an object that is no longer required remains alive in the JVM memory. There are various reasons for memory leaks in Java, which are also responsible for creating different types of memory leaks in Java applications. Memory leaks lead to poor application performance over time because of your application’s increased (and unexpected) use of memory and resources.

It should be noted that the garbage collector is adept at collecting objects that are not being referenced, periodically. However, it does not collect those objects that are still in use, i.e., objects that still have references. Here is exactly where memory leaks occur. To help prevent memory leaks, it is important to design programs in such a way that they release memory when it is no longer needed.

Additionally, programmers should be aware of the potential for memory leaks and be sure to test their programs thoroughly for them. When a memory leak occurs, the program will slowly use up more and more memory until it eventually crashes.

Read: Tips to Improve Performance in Java Applications

What are Memory Leaks in Stack and Heap?

In Java, you might encounter memory leaks both for stack, as well as, heap memory. Heap memory leaks occur when an object is created but never removed from the heap. This can happen if the code is referencing an object that is no longer needed but the reference is never removed. Eventually, the heap will fill up with unused objects and the application will soon run out of memory.

Stack memory leaks occur when a method keeps getting called but never exits. This can happen if there is an infinite loop or if the method is being called with different data each time but the data is never used. Eventually, the stack will fill up and the program will run out of memory.

Why do Memory Leaks Occur in Java?

One of the most common causes of memory leaks in Java is when an application fails to properly release resources that it no longer needs. This can happen when an application allocates memory for objects but never releases them, even when they’re no longer needed. When this happens, the memory that was allocated for those objects is never freed, and the application’s overall memory usage gradually increases over time.

This can happen due to programming errors, or simply because the programmer forgot to include code that would do so. In either case, it can result in decreased performance for the affected program, and in worst-case scenarios, can cause the program to crash.

Memory leaks can occur due to programming errors, i.e., when you acquire memory but do not release them when the same memory is no longer needed. To fix this, you should write the necessary code to release the acquired memory.

Read: A Guide to Writing Garbage Collection in Java

How to Prevent Memory Leaks in Java

One of the best ways to prevent memory leaks in Java is to use a tool like JProfiler, which can help you identify places where your code is allocating memory but not releasing it properly. JProfiler can also help you identify other potential causes of memory leaks, such as holding on to references to objects that are no longer needed.

Once you have identified potential sources of memory leaks, you can then modify your code to make sure that resources are properly released when they’re no longer needed. This can help reduce the overall memory footprint of your application and improve its overall performance.

Static field, Instance Field, and Thread Local Memory Leaks

There are several types of memory leaks programmers can encounter in Java applications. Leaks caused by static fields are the most common. This occurs when you use a static field in a class that is no longer needed.

An “instance field” leak occurs when an instance field of a class references an object that is no longer needed. A “thread local” leak occurs when a thread local variable references an object instead of sharing the instance across multiple threads, till the thread is alive.

How to Avoid Memory Leaks in Java: Best Practices

You can adhere to the following best practices to avoid memory leaks in Java:

  • Use the latest version of Java.
  • Use a memory leak detector tool like Eclipse MAT, Visual VM, JProbe, YourKit, JRockit, AppPerfect, and so on.
  • Use a profiler tool to detect memory leaks.
  • Avoid using System.gc() method calls in your code
  • Use a memory leak detection tool such as YourKit, JProfiler or etc
  • Avoid using inner classes.

Memory leaks can be difficult to diagnose and fix. However, by understanding the causes, you can detect and fix them. You should be able to find the root cause of any memory leak by following these steps:

  • Identify that a memory leak exists (i.e., with a profiler or a performance monitor)
  • Check the call stack for references that have not been released
  • Determine why such objects haven’t been garbage collected
  • Write your code to remove such objects manually

Final Thoughts on Memory Leaks in Java

While a memory leak occurs when a program fails to release unused resources, such as objects, database connections, or file handles, a reference leak occurs when an object keeps a reference to another object, even after the latter is no longer needed. These unintended object references prevent the built-in Java garbage collection process from clearing the memory these objects acquire. In this article, we looked at what a memory leak is, why it occurs and how you can prevent memory leaks in Java.

Read more Java programming and software development tutorials.

Source: www.developer.com