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2.13. Public Design, Private Implementation

Thus far this specification has sketched the public view of the Java Virtual Machine: the class file format and the instruction set. These components are vital to the hardware-, operating system-, and implementation-independence of the Java Virtual Machine. The implementor may prefer to think of them as a means to securely communicate fragments of programs between hosts each implementing the Java SE platform, rather than as a blueprint to be followed exactly.

It is important to understand where the line between the public design and the private implementation lies. A Java Virtual Machine implementation must be able to read class files and must exactly implement the semantics of the Java Virtual Machine code therein. One way of doing this is to take this document as a specification and to implement that specification literally. But it is also perfectly feasible and desirable for the implementor to modify or optimize the implementation within the constraints of this specification. So long as the class file format can be read and the semantics of its code are maintained, the implementor may implement these semantics in any way. What is “under the hood” is the implementor’s business, as long as the correct external interface is carefully maintained.

  • There are some exceptions: debuggers, profilers, and just-in-time code generators can each require access to elements of the Java Virtual Machine that are normally considered to be “under the hood.” Where appropriate, Oracle works with other Java Virtual Machine implementors and with tool vendors to develop common interfaces to the Java Virtual Machine for use by such tools, and to promote those interfaces across the industry.

The implementor can use this flexibility to tailor Java Virtual Machine implementations for high performance, low memory use, or portability. What makes sense in a given implementation depends on the goals of that implementation. The range of implementation options includes the following:

  • Translating Java Virtual Machine code at load-time or during execution into the instruction set of another virtual machine.
  • Translating Java Virtual Machine code at load-time or during execution into the native instruction set of the host CPU (sometimes referred to as just-in-time, or JIT, code generation).

The existence of a precisely defined virtual machine and object file format need not significantly restrict the creativity of the implementor. The Java Virtual Machine is designed to support many different implementations, providing new and interesting solutions while retaining compatibility between implementations.

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