What is the compilation order of java code

java code compilation sequence: first the source code enters the lexical analyzer, and then the token stream enters the syntax analyzer; then the abstract syntax tree is processed and then the semantic analyzer is performed; then the abstract syntax tree is annotated and generated using bytecode processor; finally generates jvm bytecode.

Java code compilation sequence:

Java code compilation is completed by the Java source code compiler. The flow chart is as follows:

The execution of Java bytecode is completed by the JVM execution engine. The flow chart is as follows:

The entire process of Java code compilation and execution includes the following three important mechanisms: 1) Java source code compilation mechanism; 2) Class loading mechanism; 3) Class execution mechanism

Among them, Java source code compilation consists of the following three processes: 1) analysis and input to the symbol table; 2) annotation processing; 3) semantic analysis and generation of class files

The flow chart is as follows:

The finally generated class file consists of the following parts:

1) Structural information. Includes class file format version number and information on the number and size of each part.

2) Metadata. Corresponds to information about declarations and constants in Java source code. Contains declaration information of classes, inherited superclasses, implemented interfaces, domain and method declaration information, and constant pools.

3) Method information. Corresponds to the information corresponding to statements and expressions in Java source code. Contains bytecode, exception handler table, evaluation stack and local variable area size, evaluation stack type record, and debugging symbol information.

Class loading mechanism

JVM class loading is completed through ClassLoader and its subclasses. The hierarchical relationship and loading order of the classes are as shown in the figure below:

1) BootStrap ClassLoader

is responsible for loading all classes in the jre/lib/rt.jar package in $JAVA_HOME. It is implemented by C and is not a ClassLoader subclass

2) Extension ClassLoader

is responsible for loading some jar packages for extension functions in the Java platform, including jars in the directory specified by jre/lib/*.jar or -Djava.ext.dirs in $JAVA_HOME Bag.

3) App ClassLoader

is responsible for recording the jar package specified in the classpath and the class in the directory

4) Custom ClassLoader

belongs to the application according to its own needs Customized ClassLoaders, such as tomcat and jboss, will implement ClassLoader according to j2ee specifications.

During the loading process, it will first check whether the class has been loaded. The checking order is from bottom to top, from Custom ClassLoader to BootStrap. ClassLoader checks layer by layer. As long as a certain classloader has been loaded, the class is considered loaded, ensuring that this class is only loaded once. The loading order is from top to bottom.

Class execution mechanism

JVM is based on the stack architecture to execute class bytecode. After a thread is created, a program counter (PC) and a stack (Stack) will be generated. The program counter stores the next instruction to be executed. Each stack frame is stored in the stack. Each stack frame corresponds to each call of each method. The stack frame is composed of two parts: the local variable area and the operand stack. The local variable area is used to store local variables in the method. and parameters, the operand stack is used to store intermediate results generated during method execution. The structure of the stack is as follows:

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