The Java Virtual Machine / Exokernels

The Java Virtual Machine / Exokernels

The Java Virtual Machine

One more area where virtual machines are used, but in a somewhat different way, is for running Java programs. When Sun Microsystems invented the Java programming language, it also invented a virtual machine (i.e., a computer architecture) called the JVM (Java Virtual Machine). The Java compiler produces code for JVM, which then normally is executed by a software JVM interpreter. The advantage of this technique is that the JVM  code can be shipped over the Internet to any computer that has a JVM interpreter and run there. If the compiler had produced SPARC or Pentium binary programs, for instance, they could not have been shipped and run anywhere as easily. (Of course, Sun could have produced a compiler that produced SPARC binaries and then distributed a SPARC interpreter, but JVM is a much simpler architecture to interpret.) Another advantage of using JVM is that if the interpreter is implemented properly, which is not completely trivial, incoming JVM programs can be checked for safety and then carried out in a protected environment so they cannot steal data or do any damage.


Instead of cloning the actual machine, as is done with virtual machines, another approach is partitioning it, in other words, giving each user a subset of the resources. In this way one virtual machine might get disk blocks 0 to 1023, the next one might get blocks 1024 to 2047, and so on.

At the bottom layer, running in kernel mode, is a program called the exokernel (Engler et al., 1995). Its job is to distribute resources to virtual machines and then check attempts to use them to make sure no machine is trying to use somebody else's resources. Each user-level virtual machine can run its own operating system, as on VM/370 and the Pentium virtual  8086s, except that each one is restricted to using only the resources it has asked for and been allocated.

The advantage of the exokernel scheme is that it saves a layer of mapping. In the other designs, each virtual machine thinks it has its own disk, with blocks running from 0 to some maximum, so the virtual machine monitor must maintain tables to remap disk addresses (and all other resources). With the exokernel, this remapping is not required. The exokernel require only keep track of which virtual machine has been assigned which resource. This technique still has the advantage of separating the multiprogramming (in the exokernel) from the user operating system code (in user space), but with less overhead, since all the exokernel has to do is keep the virtual machines out of each other's hair.


virtual machine, kernel mode, multiprogramming