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
复制代码对于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
复制代码对于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
复制代码执行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.
复制代码可以看到对于mac的jdk12，把client设置为default则报错
禁用client
-server KNOWN
-client ERROR
复制代码执行java -client -version显示如下
java -client -version
Error: client VM not supported
复制代码可以看到设置-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'
复制代码可以看到删除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