Summary of class file format in Java

This article mainly introduces the relevant information of Java class file format summary. It is very good and has reference value. Friends who need it can refer to it

We all know that the JVM can only recognize files in class format, and source files are only recognized by us and cannot be recognized by the JVM. Then we need to understand the Java language and JVM at a deeper level. It is not enough to only understand the source files, because many behaviors of the virtual machine are defined in class files, and we need to understand the JVM To behave differently, you must also learn the "language" that the JVM can understand, which is the class file format. Just like we want to understand a foreigner deeply, it is impossible to understand him only from our own perspective. Only if you learn his language can you understand him better, because only if you understand what he says can you understand him better. Know what he does, and then understand his way of doing things and his personality traits. Of course, there is another way for you to understand foreigners, that is, let them learn Chinese and you can communicate in Chinese. But in the Java world, this is impossible, because you cannot let the JVM recognize the source file, so you can only learn the language that the JVM can understand ---- class file.

Understanding the format of the class file not only has guiding significance for understanding the behavior of the JVM, it can also give us a clearer and deeper understanding of the Java code. After all, the class file is the java code written by us. "Translated". After understanding the format of the class file, you can know how the method is represented, how the fields are represented, how inheritance is represented, how the method body is represented, etc. All things in the source file will have corresponding description forms in the class file. This will allow us to be confident when writing code. When there is a problem with the code, we can find out the cause faster because we can decompile the class file generated by the source code and see how it is implemented internally. After all, the principle is clear, the source code is just a representation.

In addition, class files can not only be generated by Java code. Many other JVM-based languages ​​also generate standard class files, which are then executed by the virtual machine. For example, the scala language I am learning recently is based on virtual machines. The scalac compiler can compile scala code into class files. In this case, understanding the class file format can play a good role in learning other JVM-based languages. You will find that if you know enough about class files and JVM, learning these languages ​​will be very easy. A class file is an interface of the JVM. No matter what code you write in the source file, whether it is java or scala, as long as you use a certain method, the code you write can be converted into a class file with the correct format. , then the JVM can execute it for you. If you are good enough, you can create a language by yourself, implement a compiler for this language, compile it into a class, and then the virtual machine can also execute it, and designers of Scala, Groovy and other languages ​​are already doing this.

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