JVM中MemoryUsage中init,committed,used,max的含義

以下摘抄自JDK1.7

 * <table>
 * <tr>
 * <td valign=top> <tt>init</tt> </td>
 * <td valign=top> represents the initial amount of memory (in bytes) that
 *      the Java virtual machine requests from the operating system
 *      for memory management during startup.  The Java virtual machine
 *      may request additional memory from the operating system and
 *      may also release memory to the system over time.
 *      The value of <tt>init</tt> may be undefined.
 * </td>
 * </tr>
 * <tr>
 * <td valign=top> <tt>used</tt> </td>
 * <td valign=top> represents the amount of memory currently used (in bytes).
 * </td>
 * </tr>
 * <tr>
 * <td valign=top> <tt>committed</tt> </td>
 * <td valign=top> represents the amount of memory (in bytes) that is
 *      guaranteed to be available for use by the Java virtual machine.
 *      The amount of committed memory may change over time (increase
 *      or decrease).  The Java virtual machine may release memory to
 *      the system and <tt>committed</tt> could be less than <tt>init</tt>.
 *      <tt>committed</tt> will always be greater than
 *      or equal to <tt>used</tt>.
 * </td>
 * </tr>
 * <tr>
 * <td valign=top> <tt>max</tt> </td>
 * <td valign=top> represents the maximum amount of memory (in bytes)
 *      that can be used for memory management. Its value may be undefined.
 *      The maximum amount of memory may change over time if defined.
 *      The amount of used and committed memory will always be less than
 *      or equal to <tt>max</tt> if <tt>max</tt> is defined.
 *      A memory allocation may fail if it attempts to increase the
 *      used memory such that <tt>used > committed</tt> even
 *      if <tt>used <= max</tt> would still be true (for example,
 *      when the system is low on virtual memory).
 * </td>
 * </tr>
 * </table>
 * </ul>
 *
 * Below is a picture showing an example of a memory pool:
 * <p>
 * <pre>
 *        +----------------------------------------------+
 *        +////////////////           |                  +
 *        +////////////////           |                  +
 *        +----------------------------------------------+
 *
 *        |--------|
 *           init
 *        |---------------|
 *               used
 *        |---------------------------|
 *                  committed
 *        |----------------------------------------------|
 *                            max
 * </pre>

實驗：

import java.util.ArrayList;
import java.util.List;


public class VMTest {
	public static void main(String[] args) {
		List<Test> l = new ArrayList<Test>();
		while(true) {
			try {
				Thread.sleep(1);
				l.add(new Test());
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}

}
class Test {
	int[] a = new int[2560];
}

相當於每秒鐘分配10M內存。
啓動參數 -Xmx512m -Xms10m

結果：

init:10485760 略小於xms的10m

max:477233152 略小於xmx的512m

committed和used不停地增大，used始終小於committed,40幾秒後達到max
,報java.lang.OutOfMemoryError: Java heap space錯誤。

結論：init約等於xms的值，max約等於xmx的值。used是已經被使用的內存大小，committed是當前可使用的內存大小（包括已使用的），committed >= used。committed不足時jvm向系統申請，若超過max則發生OutOfMemoryError錯誤。