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

对于jdk8该文件在JAVA_HOME/jre/lib/目录下；其注释显示该配置文件用于配置java、javac能够使用的option，其中配置顺序非常重要，第一个为default JVM(不管其是KNOWN还是IGNORE；IGNORE仅仅是禁用VM option；ERROR则报错显示not supported)；可以看到这里-server是默认的，而-client则被忽略

jdk9-12

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

-server KNOWN
-client IGNORE

对于jdk9、10、11、12该文件在JAVA_HOME/lib/目录下

设置-client为default

对于jdk12，把-client移到前面

-client IGNORE
-server KNOWN

执行java -version显示如下

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.

可以看到对于mac的jdk12，把client设置为default则报错

禁用client

-server KNOWN
-client ERROR

执行java -client -version显示如下

java -client -version
Error: client VM not supported

可以看到设置-client为ERROR，则报错显示not supported

删除jvm.cfg

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

可以看到删除jvm.cfg，执行java -version则会报错

小结

• jvm.cfg文件用于配置java、javac能够使用的option，其中配置顺序非常重要，第一个为default JVM(不管其是KNOWN还是IGNORE；IGNORE仅仅是禁用VM option；ERROR则报错显示not supported)
• 对于jdk8该文件在JAVA_HOME/jre/lib/目录下；对于jdk9、10、11、12该文件在JAVA_HOME/lib/目录下；删除jvm.cfg文件的话，则执行java命令会报错
• 可以看到对于mac来说其open jdk的jvm.cfg中-server位于-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