several ways to monitor the jvm

using jconsole in jdk1.5

• see http://java.sun.com/j2se/1.5.0/docs/guide/management/overview.html
• or http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html
• add -Dcom.sun.management.jmxremote to vm paras
• start /bin/jconsole

Runtime.getRuntime().freeMemory() returns current freeMemory

using -Xloggc: para

• add '-Xloggc:gc.log' to vm parameters and the file gc.log shows detail memory usage from startup
• there is many tools to analyze the file to show you figures.