What Are Memory Leaks in Java? And How to Find Them

Optimize your application’s performance by effectively managing resources, especially memory resources. Prevent, identify, or eliminate memory leaks within your java applications before they affect your application, business, and the user experience.

What causes memory leaks in java?

The ingrained memory management tool in Java, known as Java Virtual Machine or JVM, handles the process of garbage collection, which includes finding and removing unused objects in order to efficiently manage memory allocation. However, the garbage collector can only detect and remove unreferenced java objects, so memory leaks may still occur in java applications when object references are no longer needed but are still being maintained. 

Common scenarios that lead to memory leaks include:

Static fields and collections

The life cycle of a static variable exists through the duration of the application, remaining in memory until the application ends. Since static fields and collections are excluded from garbage collection and require deliberate clean-up, they are often the culprit of memory leaks. Mutable static fields should be avoided during programming.

Thread-local variables:

Similar to a static variable, thread local variables are overlooked by the garbage collector due to the thread being considered active as a member in the thread class. If threads are continuously pooled and kept alive, garbage collection will ignore the associated thread-local variables, contributing to a potential memory leak.

Complex and circular object structures:

Circular or complex bi-directional references might be too complicated for the garbage collector to detect. These leaks can also be particularly difficult to track down without a thorough understanding of each piece of code or a heap analysis; sometimes the existence of an element such as a child object can prevent an associated document or nodes from being removed even if they were deleted.

Java Native Interface (JNI) memory leaks:

Native code from Java has the ability to call, handle, and create Java objects. Since the Java objects created using the native method originate as a local reference, they are often changed into a global reference or are referenced by another Java object so that they can exist once the native call ends. However, in doing so, they are exempt from the collection process and eventually create problems that can be difficult to trace back to the source of the issue. 

How to find memory leaks

Java memory leaks can result in fatal application crashes if they are not identified and addressed promptly.  Monitor memory usage and mitigate Java application memory leaks with the following techniques and profiling tools:

Common scenarios to find memory leaks include:

OutOfMemoryerror message

One very common sign of memory leaks is the java.lang.OutOfMemoryError message. Investigate further for detailed messages that may provide additional clues. For instance: 

  • Java heap space may indicate a memory leak, that the specified heap size is too low, or that your finalizer queue is overloaded. 

  • PermGen space means that the area that stores method and class objects is full. This can be remedied by using -XX:MaxPermSize to increase the space. 

  • Requested array size exceeds VM limit is evidence that an array that is greater than the heap size is being assigned. 

  • <Reason> <stack trace> (Native method) indicates that a native method was unable to be allocated the necessary amount of memory. 

  • Request <size> bytes for <reason>. Out of swap space? is a notice that the native heap is near capacity or that a local heap allocation failed. 

Verbose garbage collection

Enabling verbose garbage collection is an effective and efficient way to detect a memory leak. For a detailed look at garbage collection, add the -verbose:gc parameter to the JVM configuration of your Java application to view summary reports that provide a better understanding of memory utilization.

Profiling and monitoring

Identify unusual memory consumption with a low overhead memory monitoring tool such as AppDynamics.

A memory profiler such as VisualVM is another option that takes advantage of features that surpass command-line JDK tools. A Java profiler can help detect memory leaks by providing meaningful data that can facilitate further analysis.

How to prevent memory leaks

Thoroughly understanding and carefully reviewing your application code is a programming best practice that has benefits that extend beyond preventing or identifying memory leaks. Avoid shortcuts that could undermine your efforts in the future.

Consider the following during the programming process to reduce the potential for memory leaks:

  • Use special reference objects to refer to new objects indirectly so that they will be cleared by garbage collection
  • Use preventers to mitigate a range of problems associated with WebApp classloaders
  • Terminate the session with HttpSession.invalidate() when it's no longer in use
  • Set the session time-out time to a low setting
  • Don't store unnecessary data in HttpSession
  • Use the page directive <%@page session=”false”%> to avoid creating HttpSession in your jsp page, when possible 
  • Improve performance by using StringBuffer's append () method to avoid the unnecessary objects created by string concatenation
  • Use the pre-compiled PreparedStatement versus Statement object for frequently executed queries
  • Close the Statement and ResultSet prior to reuse, and close ResultSet, Connection, PreparedStatement, and Statement in the finally block
  • Use the column name instead of "*" when writing queries in JDBC code
  • Be sure to close a loop if using stmt = con.prepareStatement(sql query)within that loop

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