最近在開發Java的程序。本來我是一直很喜歡Java的內存管理的，不需要擔心分配內存，只管分配，垃圾收集器自己會給你回收內存的?，F在開發的程序數據量很大，為了速度快，我準備把所有的信息加載進內存，這樣可以保證快速響應。我還在反復算內存，想想自己的數據量，現在剛開始的時候應該夠了（我的機器是4G內存，雖然Windows就認3.5G，但是比起我現在的數據量應該沒問題）。
沒想到第一個實驗的程序，跑了幾個小時，就遇到了Out of Memory Exception了?？纯醋约旱奶摂M機設置，我設置的是-Xms512M -Xmx1024M。想都沒想，直接改成-Xms512M -Xmx2048M，結果直接就Could not reserve enough space for object heap。程序都起不來了。這才發現原來最大內存還有限制。上網搜了一下干洗機，發現很多討論這個問題的文章。最終在BEA的DEV2DEV論壇發現了最有用的一篇http://dev2dev.bea.com.cn/bbs/thread.jspa?forumID=121&threadID= 35704&start=0&tstart=0
這里的版主YuLimin 做了測試，得出結論：
公司 JVM版本 最大內存(兆)client 最大內存(兆)server
SUN 1.5.x 1492 1520
SUN 1.5.5(Linux) 2634 2660
SUN 1.4.2 1564 1564
SUN 1.4.2(Linux) 1900 1260
IBM 1.4.2(Linux) 2047 N/A
BEA JRockit 1.5 (U3) 1909 1902
我現在用的是JDK1.6. 0_05，測試了一下。在Client狀態下最大是，我的JDK不認-Server參數，測試不了Server狀態。估計差不多。
SUN 1.6.0 1442 N/a
看樣子用Java想用大內存也是不可能的了。而且一般的說法是內存太大了，垃圾收集的時間就會長。這也可以理解，一般是內存不夠用了才收集的，掃描2G內存比1G當然要慢多了，而且內存對象多了，估計關系是指數上升的。
下面附上YuLimin的測試方法和測試記錄。
測試方法：在命令行下用 java -XmxXXXXM -version 命令來進行測試，然后逐漸的增大XXXX的值，如果執行正常就表示指定的內存大小可用，否則會打印錯誤信息。
測試記錄：
我在Windows 2000 ADS上面測試內存使用的結果如下
SUN的1.2.2、1.3.1、1.4.2、1.5.0、IBM1.4.2、BEA JRockit 1.4.2
F:\JDK\1.2.2\bin>java -Xmx700000255M -version
java version “1.2.2″
Classic VM (build JDK-1.2.2_017, native threads, symcjit)
F:\JDK\1.2.2\bin>java -Xmx700000256M -version
Bad max heap size: -Xmx700000256M
Could not create the Java virtual machine.
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F:\JDK\1.3.1\bin>java -version
java version “1.3.1_18″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_18-b01)
Java HotSpot(TM) Client VM (build 1.3.1_18-b01, mixed mode)
F:\JDK\1.3.1\bin>REM If present, the option to select the VM must be first.
F:\JDK\1.3.1\bin>REM The default VM is -hotspot.
F:\JDK\1.3.1\bin>java -hotspot -Xmx1554M -version
java version “1.3.1_18″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_18-b01)
Java HotSpot(TM) Client VM (build 1.3.1_18-b01, mixed mode)
F:\JDK\1.3.1\bin>java -hotspot -Xmx1555M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
F:\JDK\1.3.1\bin>java -server -Xmx1522M -version
java version “1.3.1_18″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_18-b01)
Java HotSpot(TM) Server VM (build 1.3.1_18-b01, mixed mode)
F:\JDK\1.3.1\bin>java -server -Xmx1523M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
F:\JDK\1.3.1\bin>java -classic -Xmx2047M -version
java version “1.3.1_18″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_18-b01)
Classic VM (build 1.3.1_18-b01, native threads, nojit)
F:\JDK\1.3.1\bin>java -classic -Xmx2048M -version
Bad max heap size: -Xmx2048M
Could not create the Java virtual machine.
=====================================================================
F:\JDK\1.4.2\bin>java -version
java version “1.4.2_12″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_12-b03)
Java HotSpot(TM) Client VM (build 1.4.2_12-b03, mixed mode)
F:\JDK\1.4.2\bin>REM The default VM is client.
F:\JDK\1.4.2\bin>java -client -Xmx1308M -version
java version “1.4.2_12″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_12-b03)
Java HotSpot(TM) Client VM (build 1.4.2_12-b03, mixed mode)
F:\JDK\1.4.2\bin>java -client -Xmx1309M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
F:\JDK\1.4.2\bin>java -server -Xmx1308M -version
java version “1.4.2_12″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_12-b03)
Java HotSpot(TM) Server VM (build 1.4.2_12-b03, mixed mode)
F:\JDK\1.4.2\bin>java -server -Xmx1309M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
F:\JDK\1.4.2\bin>REM -hotspot is a synonym for the “client” VM [deprecated]
F:\JDK\1.4.2\bin>java -hotspot -Xmx1308M -version
java version “1.4.2_12″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_12-b03)
Java HotSpot(TM) Client VM (build 1.4.2_12-b03, mixed mode)
F:\JDK\1.4.2\bin>java -hotspot -Xmx1309M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
F:\JDK\1.5.0\bin>java -version
java version “1.5.0_07″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-b03)
Java HotSpot(TM) Client VM (build 1.5.0_07-b03, mixed mode, sharing)
F:\JDK\1.5.0\bin>java -client -Xmx1492M -version
java version “1.5.0_07″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-b03)
Java HotSpot(TM) Client VM (build 1.5.0_07-b03, mixed mode)
F:\JDK\1.5.0\bin>java -client -Xmx1493M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
F:\JDK\1.5.0\bin>java -server -Xmx1504M -version
java version “1.5.0_07″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-b03)
Java HotSpot(TM) Server VM (build 1.5.0_07-b03, mixed mode)
F:\JDK\1.5.0\bin>java -server -Xmx1505M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
F:\JDK\1.5.0\bin>REM -hotspot is a synonym for the “client” VM [deprecated]
F:\JDK\1.5.0\bin>java -hotspot -Xmx1492M -version
java version “1.5.0_07″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-b03)
Java HotSpot(TM) Client VM (build 1.5.0_07-b03, mixed mode)
F:\JDK\1.5.0\bin>java -hotspot -Xmx1493M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
=====================================================================
F:\JDK\IBM142\bin>java -version
java version “1.4.2″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2)
Classic VM (build 1.4.2, J2RE 1.4.2 IBM Windows 32 build cn1420-20040626 (JIT enabled: jitc))
F:\JDK\IBM142\bin>REM The default VM is client.
F:\JDK\IBM142\bin>java -Xmx2047M -version
java version “1.4.2″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2)
Classic VM (build 1.4.2, J2RE 1.4.2 IBM Windows 32 build cn1420-20040626 (JIT enabled: jitc))
F:\JDK\IBM142\bin>java -Xmx2048M -version
[ Unable to allocate an initial java heap of 2147483648 bytes. ]
[ **Out of memory, aborting** ]
[ ]
[ *** panic: JVMST016: Cannot allocate memory for initial java heap ]
abnormal program termination
F:\BEA\JRockit\bin>java -version
java version “1.4.2_05″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-b04)
BEA WebLogic JRockit(TM) 1.4.2_05 JVM R24.4.0-1 (build ari-38120-20041118-1131-win-ia32, Native Threads, GC strategy: parallel)
F:\BEA\JRockit\bin>REM The default VM is jrockit.
F:\BEA\JRockit\bin>java -Xmx1617M -version
java version “1.4.2_05″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-b04)
BEA WebLogic JRockit(TM) 1.4.2_05 JVM R24.4.0-1 (build ari-38120-20041118-1131-win-ia32, Native Threads, GC strategy: parallel)
F:\BEA\JRockit\bin>java -Xmx1618M -version
Unable to acquire some virtual address space - reduced from 1656832 to 1640260 KB!
java version “1.4.2_05″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-b04)
BEA WebLogic JRockit(TM) 1.4.2_05 JVM R24.4.0-1 (build ari-38120-20041118-1131-win-ia32, Native Threads, GC strategy: parallel)
F:\BEA\JRockit\bin>REM -jrockit to select the “jrockit” VM
F:\BEA\JRockit\bin>java -jrockit -Xmx1617M -version
java version “1.4.2_05″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-b04)
BEA WebLogic JRockit(TM) 1.4.2_05 JVM R24.4.0-1 (build ari-38120-20041118-1131-win-ia32, Native Threads, GC strategy: parallel)
F:\BEA\JRockit\bin>java -jrockit -Xmx1618M -version
Unable to acquire some virtual address space - reduced from 1656832 to 1640260 KB!
java version “1.4.2_05″
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-b04)
BEA WebLogic JRockit(TM) 1.4.2_05 JVM R24.4.0-1 (build ari-38120-20041118-1131-win-ia32, Native Threads, GC strategy: parallel)
我的測試記錄：
C:\>java -client -Xmx1441M -version
java version “1.6.0_05″
Java(TM) SE Runtime Environment (build 1.6.0_05-b13)
Java HotSpot(TM) Client VM (build 10.0-b19, mixed mode)
C:\>java -client -Xmx1442M -version
java version “1.6.0_05″
Java(TM) SE Runtime Environment (build 1.6.0_05-b13)
Java HotSpot(TM) Client VM (build 10.0-b19, mixed mode)
C:\>java -client -Xmx1443M -version
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
C:\>java -server -Xmx1443M -version
Error: no `server’ JVM at `C:\Program Files\Java\jre1.6.0_05\bin\server\jvm.dll’