聊聊openjdk的jvm.cfg文件

序

本文主要研究一下openjdk的jvm.cfg文件

jdk8

/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/jvm.cfg

# List of JVMs that can be used as an option to java, javac, etc.
# Order is important -- first in this list is the default JVM.
# NOTE that this both this file and its format are UNSUPPORTED and
# WILL GO AWAY in a future release.
#
# You may also select a JVM in an arbitrary location with the
# "-XXaltjvm=<jvm_dir>" option, but that too is unsupported
# and may not be available in a future release.
#
-server KNOWN
-client IGNORE

對於jdk8該文件在JAVA_HOME/jre/lib/目錄下；其註釋顯示該配置文件用於配置java、javac可以使用的option，其中配置順序很是重要，第一個爲default JVM( 無論其是KNOWN仍是IGNORE；IGNORE僅僅是禁用VM option；ERROR則報錯顯示not supported)；能夠看到這裏-server是默認的，而-client則被忽略

jdk9-12

/Library/Java/JavaVirtualMachines/jdk-12.jdk/Contents/Home/lib/jvm.cfg

-server KNOWN
-client IGNORE

對於jdk九、十、十一、12該文件在JAVA_HOME/lib/目錄下

設置-client爲default

對於jdk12，把-client移到前面

-client IGNORE
-server KNOWN

執行java -version顯示以下

java -version
Error: missing `client' JVM at `/Library/Java/JavaVirtualMachines/jdk-12.jdk/Contents/Home/lib/client/libjvm.dylib'.
Please install or use the JRE or JDK that contains these missing components.

能夠看到對於mac的jdk12，把client設置爲default則報錯

禁用client

-server KNOWN
-client ERROR

執行java -client -version顯示以下

java -client -version
Error: client VM not supported

能夠看到設置-client爲ERROR，則報錯顯示not supported

刪除jvm.cfg

java -version
Error: could not open `/Library/Java/JavaVirtualMachines/jdk-12.jdk/Contents/Home/lib/jvm.cfg'

能夠看到刪除jvm.cfg，執行java -version則會報錯

小結

• jvm.cfg文件用於配置java、javac可以使用的option，其中配置順序很是重要，第一個爲default JVM(無論其是KNOWN仍是IGNORE；IGNORE僅僅是禁用VM option；ERROR則報錯顯示not supported)
• 對於jdk8該文件在JAVA_HOME/jre/lib/目錄下；對於jdk九、十、十一、12該文件在JAVA_HOME/lib/目錄下；刪除jvm.cfg文件的話，則執行java命令會報錯
• 能夠看到對於mac來講其open jdk的jvm.cfg中-server位於-client前面，-server是default JVM，而-client被忽略

doc

• The Java HotSpot VM Architecture Overview
• What is the purpose of jvm.cfg file in relation to Java?
• Running the JVM in server mode
• Server-Class Machine Detection
• Change Default Java VM to Client
• OpenJDK default options to always use the server VM