What are the benefits of compiling and running java?

Java code is compiled and run in the JVM, so what are the benefits of this running method?

Different from the C/C language, which can directly generate a code format that the CPU can understand after compilation, commonly known as machine code, Java compiles the code into words executable by the virtual machine. section code. The Java virtual machine can be implemented by hardware or by software according to different platforms (Windows, Linux, Mac, etc.). The advantage of this is that once the Java code is compiled into bytecode, it can run on different virtual machine platforms. This is what we often call "compile once, run anywhere." In addition, the virtual machine also provides dynamic detection such as hosting environment, array out-of-bounds, dynamic type, security permissions, etc., allowing us to pay more attention to the development of business logic.

Java Virtual Machine

From the perspective of the virtual machine, executing Java code first requires loading the compiled class file into the Java virtual machine. The loaded Java class will be stored in the method area. The Java virtual machine divides the runtime memory area into five parts, namely method area, heap, PC register, Java method stack and local method stack.

Java running process

During the running process, whenever a Java method is called, the Java virtual machine generates a stack frame in the Java method stack of the current thread. Swimsuit holds local variables and bytecode operands. The size of this frame is calculated in advance, and the Java virtual machine does not require the stack memory to be contiguous. When exiting a method, whether it returns normally or abnormally, the Java virtual machine will pop up the current frame and discard it. From a hardware perspective, Java bytecode cannot be run directly, so the Java virtual machine needs to translate the bytecode into machine code that the CPU can recognize.
There are two situations in the above translation process: the first is interpretation and execution, that is, translating the bytecode into machine code one by one and executing it; the second is just-in-time compilation (JIT), which converts one All bytecodes in the method are translated into machine code before execution.

The advantage of the former is that there is no need to wait for compilation, and the advantage of the latter is that it actually runs faster. In order to achieve faster running speed, Java virtual machines usually adopt two methods, that is, mixed mode, which combines the advantages of both. Just-in-time compilation technology is based on the 80-20 rule, that is, 20% of the code takes up 80% of the computing resources. Interpretation is used for most of the uncommon codes, and just-in-time compilation is used for a small number of commonly used codes, achieving a reasonable running speed.

When computing resources are sufficient, bytecode interpretation, execution, instant compilation and simultaneous execution. Once the compilation is completed, the interpretation execution can be replaced next time and the machine code can be run directly. In general, the execution efficiency of Java code has a lot to do with the configuration and operation mode of the virtual machine. Optimizing the Java virtual machine can effectively improve the running efficiency of the code.

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