问题
What do two different Java programs, running in one same Java Virtual Machine, share together?
Something comes to my mind, regarding singleton classes: They can only be instantiated once per JVM. This means that if two different Java programs use one same class, they are both accessing the same instance of that Singleton class?
What other things are shared?
回答1:
If you run the java command multiple times, you get multiple instances of the JVM. In theory these are entirely separate with nothing shared, but some implementations of the JVM (now or in the past) might attempt to share some portions of the data.
I haven't looked at it recently, but about 10 years back there was a version of the Sun JVM that would attempt to "share" .class file images between JVM instances. (This was supposedly useful on servers where there might be dozens of JVMs running at once and they might come and go fairly rapidly.)
I'm not recalling any other data shared, but there may have been more, or other data may have been added since. (Though there really isn't much else that can be shared between JVMs.)
But again, in theory this should all be "transparent" and not affect the operation of an individual JVM instance.
There was also a provision for "warm started" JVMs that were partially loaded prior to the final command. This did require some configuration on the part of the system admin, but was otherwise transparent.
回答2:
Two or more programs running in the same JVM shares everything. Regarding singleton, when written properly there will be one single instance of the class, which can be accessed by all the other objects.
来源：https://stackoverflow.com/questions/30249512/what-is-shared-in-a-jvm