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This article mainly introduces "what is the content of the Java class loading mechanism". In the daily operation, I believe many people have doubts about the content of the Java class loading mechanism. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful for you to answer the doubts about "what is the content of the Java class loading mechanism?" Next, please follow the editor to study!

One: preface

The types of the Java language can be divided into two categories: basic types and reference types.

The basic types include byte,short,int,long,float,double,boolean,char.

Reference types include classes, interfaces, array classes, and generic parameters. Because generic parameters are erased during compilation, there are actually only the first three types of Java virtual machines. In classes, interfaces, and array classes, array classes are generated directly by the Java virtual machine, while the other two have corresponding byte streams.

Two: load

1. Loading is the process of finding a byte stream and generating a java.lang.Class object that represents this class. Note that the byte stream does not have to be obtained from a Class file, which can be read from the ZIP package (for example, from the jar package and the war package), calculated and generated at run time (dynamic proxy), or generated by other files (such as converting the JSP file to the corresponding Class class). The loaded information is stored in the method area of JVM.

two。 For array classes, there is no corresponding byte stream, but is generated directly by the Java virtual machine. For other classes, the Java virtual machine needs the help of a class loader to complete the process of finding the byte stream.

3. There are two kinds of classloaders, one is to start the classloader, and the other classloaders are subclasses of java.lang.ClassLoader. The startup class loader is implemented by C++, and there is no corresponding Java object, so it can only be replaced by null in Java. In addition to launching the classloader, two other important classloaders are the extension classloader and the application classloader, both provided by the Java core class library. The startup class loader loads the most basic and important classes, such as the classes in the jar package under JRE's lib directory; the parent class of the extension class loader is the startup class loader, which loads relatively minor but general classes, such as the classes in the jar package under JRE's lib/ext directory; and the application class loader's parent class loader is the extension class loader, which is responsible for loading classes under the application path.

4.JVM loads classes through the parent delegation model, but we can also implement custom class loaders by inheriting java.lang.ClassLoader. When a class loader receives a class loading task, it will first give it to its parent class loader to complete, so the final loading task will be passed to the top-level startup class loader, only if the parent class loader cannot complete the loading task. Will try to execute the load task.

One benefit of using parental delegation is, for example, to load the class java.lang.Object located in the rt.jar package, no matter which loader loads the class, it is ultimately delegated to the top-level startup class loader, which ensures that using different classloaders will end up with the same Object object.

Three: link

Linking is the process of merging the created classes into the Java virtual machine so that they can be executed. It is divided into three stages: verification, preparation and parsing.

1. Verification

The purpose of this phase is to ensure that the loaded classes meet the constraints of the Java virtual machine.

two。 Prepare for

The purpose of this phase is to allocate memory for the static fields of the loaded class, that is, the memory space used to allocate these variables in the method area. The specific initialization of static fields in the Java code will take place later in the initialization phase.

3. Analysis

Until the class file is loaded into the Java virtual machine, this class cannot know the specific addresses of other classes and their methods and fields. I don't even know the address of my method or field. Therefore, whenever these members need to be referenced

The Java compiler generates a symbolic reference. In the run-time, this symbolic reference can generally be located on a specific target without ambiguity. The purpose of the parsing phase is to resolve these symbolic references to actual references. If the symbol reference

Point to an unloaded class, or a field or method of an unloaded class, and parsing will trigger the loading of that class.

Four: initialization

1. The initialization phase is the last stage of class loading, and after the previous class loading phase, all operations are dominated by JVM, except that you can customize the class loader during the loading phase. It is not until the initial stage that the actual execution of the Java program code defined in the class begins.

two。 In Java code, if you want to initialize a static field, you can assign it either directly at declaration time or in a static code block.

If a static field that is directly assigned is decorated by final and its type is a primitive type or string, the field is marked as a constant value (ConstantValue) by the Java compiler, and its initialization is done directly by the Java virtual machine. In addition, the direct assignment, as well as all the code in the static block, is placed in the same method by the Java compiler and named

.

The last step in class loading is initialization, which assigns values to fields marked with constant values, and executes the

The process of the method. The Java virtual machine ensures that the class's

Method is executed only once.

It is only after the initialization is complete that the class officially becomes executable.

The initialization of the 3.JVM specification enumeration class is triggered:

a. Initialize the main class specified by the user when the virtual machine starts

b. Initialize the target class of the new instruction when the new instruction is encountered to create a new instance of the target class

c. When an instruction to call a static method is encountered, initialize the class in which the static method is located

d. When an instruction to access a static field is encountered, initialize the class in which the static field is located

e. The initialization of the subclass triggers the initialization of the parent class

f. If an interface defines a default method, initializing the class of the interface directly or indirectly will trigger the initialization of the interface.

j. Initialize a class when you make a reflection call to a class using reflection API

h. Initialize the class of the method that the MethodHandle points to when the MethodHandle instance is called for the first time.

At this point, the study on "what is the content of the Java class loading mechanism" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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