How java code is recognized by machines

How is Java code recognized by the machine?

First of all, as a high-level programming language, Java has a very complex syntax. If you throw Java code directly to the machine, the machine will not be able to recognize the Java code. Therefore, for the Java language, we first use a compiler to compile our Java source code (.java file) into a bytecode file (.class file), and then use a virtual machine (JVM) to translate the bytecode into a machine code, and finally the machine code can be recognized and executed by the machine.

Please look at the picture below:

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Explanation As follows:

In fact, the JVM cannot run bytecode files directly. The JVM needs to translate the bytecode into machine code before it can run the corresponding code logic. In the HotSpot virtual machine, there are two forms of translation: the first is interpretation and execution, that is, translating bytecode into machine code one by one and executing it; the second is just-in-time compilation (JIT), which is All bytecode contained in a method is compiled into machine code before execution. Both methods have their own pros and cons.

The advantage of interpreting execution is that it can be run immediately without waiting for compilation. The advantage of just-in-time compilation is that it actually runs faster. The HotSpot virtual machine adopts hybrid mode by default, which combines the advantages of interpreted execution and just-in-time compilation. It will first interpret and execute the bytecode, and then compile the hotspot codes that are repeatedly executed in method units on the fly.

When the Java virtual machine executes Java bytecode, it will first load the bytecode file into memory, and then store the loaded Java class in the method area (Method Area). When actually running, the virtual machine executes the code in the method area.

During the running process, each time a Java method is entered, the Java virtual machine generates a stack frame in the Java method stack of the current thread to store local variables and bytecode operands. When exiting the current execution method, regardless of normal return or abnormal return, the Java virtual machine will pop up the current stack frame of the current thread and discard it.

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