Method Area Expansion Demonstration

JVM Tutorials - Herong's Tutorial Examples

∟Method Area Expansion Demonstration

This section provides some details of about HotSpot Method Area and an example program to demonstrate the expansion of the Method Area.

From the previous section, we learned that JVM runtime has 6 types of data areas: Method Area, Heap, Direct Memory, PC Register, JVM Stack and Native Method Stack.

Here are my understandings of the first data area, Method Area, based readings available on the Internet:

The Method Area is shared by all threads.
The Method Area contains two types of data: Classes and Runtime Constant Pool.
The Method Area will be dynamically expanded when more classes are loaded or more constants are added.
JVM garbage collector may automatically remove classes that are not used any more.
In HotSpot JVM, the Method Area is also called Permanent Generation (PermGen) space, or Class Metadata space.
In HotSpot JVM 1.7 or earlier, the size of Method Area can be controlled by "-XX:PermSize (Minimum size)" and "-XX:MaxPermSize (Maximum size)" options
In HotSpot JVM 1.8 or later, the size of Method Area can be controlled by "-XX:MetaspaceSize (Minimum size)" and "-XX:MaxMetaspaceSize (Maximum size)" options

To validate some of these understandings and demonstrate the Method Area expansion, I wrote the following sample program, MethodAreaLoadJar.java, which loads all classes from the rt.jar:

/* MethodAreaLoadJar.java
 * Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
import java.io.Console;
import java.util.*;
import java.util.jar.*;
class MethodAreaLoadJar {
   public static void main(String[] a) {
      heapCheck();
      loadRtJar();
      heapCheck();
      Runtime rt = Runtime.getRuntime();
      rt.gc();
      heapCheck();
   }
   public static void loadRtJar() {
      String jarName = System.getProperty("java.home")
         +"\\lib\\"+"rt.jar";
      System.out.println("Loading all classes in: "+jarName);
      ClassLoader loader = ClassLoader.getSystemClassLoader();
      try {
     	 JarFile jar = new JarFile(jarName); 
         Enumeration all = jar.entries();
         int count = 0;
         while (all.hasMoreElements()) {
            JarEntry one = (JarEntry) all.nextElement();
            String name = one.getName();
            if (name.indexOf(".class")>=0) {
               name = name.replaceAll(".class$", "");
               name = name.replaceAll("/", ".");
               loader.loadClass(name);
               count++;
            }
         }
         System.out.println("# of classes loaded: "+count);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
   public static void heapCheck() {
      Runtime rt = Runtime.getRuntime();
      Console con = System.console();
      long total = rt.totalMemory();
      long free = rt.freeMemory();
      long used = total - free;
      con.format("Total memory: %s%n",total);
      con.format(" Free memory: %s%n",free);
      con.format(" Used memory: %s%n",used);
      String str = con.readLine("Press ENTER key to continue: ");
   }
}

This program is designed to:

Start the JVM and stop the execution immediately to report the heap usage and to allow us to check the JVM footprint.
Then load all classes from the rt.jar provided by the JDK. Stop the execution again to report the heap usage and to allow us to check the JVM footprint.
Then call the garbage collector explicitly to see if the heap area and the method area will shrink or not.

Test results are discussed in the next section.

Last update: 2014.

Table of Contents

About This Book

Downloading and Installing JDK 1.8.0 on Windows

Downloading and Installing JDK 1.7.0 on Windows

java.lang.Runtime Class - The JVM Instance

java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

Class Class - Class Reflections

Sun's JVM - Java HotSpot VM

JRockit JVM 28.2.7 by Oracle Corporation