Learn Java (How-To): Invoking Java's garbage collector

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Using the Runtime method to invoke the garbage collector

Introduction

As you know, the Java Virtual Machine (JVM) takes care of memory management for you. That's one of the beauties of Java - life as a Java developer is easier than C/C++ because you don't have to worry (as much) about memory management, and making all of the mistakes associated with memory management, such as forgetting to free dynamically allocated memory.

Generally speaking (and ignoring the differences between implementations and versions) the JVM works by performing garbage collection when it needs more memory to continue execution. Though your program may lose a reference to an object - thereby making it a candidate to be reclaimed - there's no guarantee the JVM will reclaim that memory.

Although you can't tell Java to explicitly reclaim the memory of a specific de-referenced object, there is a way to tell the JVM to perform garbage collection, which may result in the desired memory being reclaimed. It's a very simple technique, and I was inspired to use it when I was developing a Java-based editor on a computer with limited memory itself.

Invoking the garbage collector

Invoking the garbage collector requires a simple two-step process. First you create a Java Runtime object. If you haven't used them before, Runtime objects let you interface with the environment in which your application is running. Then, after creating the Runtime object, you'll invoke the gc() method ("garbage collector") of the Runtime class.

Written in Java, these two steps look like this:

Runtime r = Runtime.getRuntime();

r.gc();

That's all that's required in this technique. Let's look at how this works in a test program.

A simple test program

Listing 1 shows a simple program that tests this technique. Although it may look a little complicated, all we do is grab a bunch of memory and then release it. At various stages in the program, we use the freeMemory() method of the Runtime class to determine the amount of free memory in the system.

// source code developed by DevDaily Interactive, Inc.
public class GCTest {
   final int NELEMS = 50000;
   void eatMemory() {
      int[] intArray = new int[NELEMS];
      for (int i=0; i<NELEMS; i++) {
        intArray[i] = i;
      }
   }
   public static void main (String[] args) {
      GCTest gct = new GCTest();
      // Step 1: get a Runtime object
      Runtime r = Runtime.getRuntime();
      // Step 2: determine the current amount of free memory
      long freeMem = r.freeMemory();
      System.out.println("free memory before creating array: " + freeMem);
      // Step 3: consume some memory
      gct.eatMemory();
      // Step 4: determine amount of memory left after consumption
      freeMem = r.freeMemory();
      System.out.println("free memory after creating array: " + freeMem);
      // Step 5: run the garbage collector, then check freeMemory
      r.gc();
      freeMem = r.freeMemory();
      System.out.println("free memory after running gc():    " + freeMem);
   }
}

Listing 1: The GCTest program (GCTest.java) demonstrates how you can manually invoke the garbage collector of the JVM.

To compile this program at the command line, just enter this command:

javac GCTest.java

Then, when you're ready to run the program, use this command to invoke the Java interpreter (the JVM):

java GCTest

Running this program with the compiler and interpreter supplied with VisualCafe 2.5 on a Windows95 computer, I get the following output:

free memory before creating array: 4054912

free memory after creating array: 3852496

free memory after running gc(): 4064184

Running the same byte code on various Unix workstations, I get different numbers, but a similar amount of memory consumption indicated by the second print statement.

As you can see, the memory initially consumed by our array in the eatMemory() method is not immediately reclaimed by the interpreter. Only after we call the gc() method is the memory reclaimed, even though the eatMemory() method has already been invoked, and control has returned to the main() method.

Download the source code

If you'd like to download the source code shown in Listing 1, just click here, and the Java source code will be displayed in your browser. Then just use the "File | Save As" option of your browser to save the source code to your system.

Listing 1:	The `GCTest` program (`GCTest.java`) demonstrates how you can manually invoke the garbage collector of the JVM.

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