Virtual machine technology is a kind of virtualization technology. The so-called virtualization technology is to change things from one form to another. The most commonly used virtualization technology is the virtualization of memory in the operating system. In actual operation, the memory space required by users may be much larger than the memory size of the physical machine. With the help of memory virtualization technology, users can virtualize some hard disks into memory, which is transparent to users.
For another example, virtual private network technology (VPN) can be used to create a safe and stable "tunnel" in the public network, and users feel like using a private network.
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The specification of the virtual machine has no special requirements for the internal structure of the object. In Sun's implementation, the reference to an object is a handle, which contains a pair of pointers: one pointer points to the method table of the object, and the other points to the data of the object. The program represented by bytecode of Java virtual machine should conform to the type rules.
The implementation of Java virtual machine should refuse to execute bytecode programs that violate the type regulations. Due to the limitation of bytecode definition, it seems that Java virtual machine can only run on machines with 32-bit address space. But you can create a Java virtual machine and automatically convert bytecode into 64-bit format. From the data types supported by Java virtual machines, it can be seen that Java strictly stipulates the internal format of data types, which makes the interpretation of data by various Java virtual machines the same, thus ensuring the platform independence and portability of Java. ?