java gc(转)

<%@ page contentType="text/html; charset=GBK"%>   
<%   
        long totalMemory = Runtime.getRuntime().totalMemory();   
  
        long freeMemory = Runtime.getRuntime().freeMemory();   
        long usedMemory = totalMemory - freeMemory;   
        java.text.NumberFormat format = new java.text.DecimalFormat("###,###");   
        String memoryInfo="垃圾回收前，内存使用情况为：已用内存/总内存：" + format.format(usedMemory) + "/" + format.format(totalMemory);   
        System.out.println(memoryInfo);   
        out.println("<br>");   
        out.println(memoryInfo);   
        out.println("<br>");   
  
		System.out.println("垃圾回收开始，时间为:"+System.currentTimeMillis());   
		out.println("垃圾回收开始，时间为:"+System.currentTimeMillis()); 
		
		System.gc();  
		
        totalMemory = Runtime.getRuntime().totalMemory();   
        freeMemory = Runtime.getRuntime().freeMemory();   
        usedMemory = totalMemory - freeMemory;   
        memoryInfo="垃圾回收前，内存使用情况为：已用内存/总内存：" + format.format(usedMemory)+ "/" + format.format(totalMemory);   
        System.out.println(memoryInfo);   
        out.println("<br>");   
        out.println(memoryInfo);   
		System.out.println("垃圾回收结束，时间为:"+System.currentTimeMillis());   
        out.println("<br>");   
		out.println("垃圾回收结束，时间为:"+System.currentTimeMillis());   
%> 

结果为：

垃圾回收前，内存使用情况为：已用内存/总内存：27,045,560/51,408,896
垃圾回收开始，时间为:1308902698673
垃圾回收前，内存使用情况为：已用内存/总内存：19,768,072/51,408,896
垃圾回收结束，时间为:1308902699023