Java class loading process

The first is the loading process (Loading), which is Java reading bytecode data from different data sources Get it into the JVM and map it to a data structure (Class object) recognized by the JVM. The data source here may be in various forms, such as jar files, class files, or even network data sources; if the input data is not a ClassFile structure, ClassFormatError will be thrown. The loading stage is the stage where users participate. We can customize the class loader to implement our own class loading process.

The second stage is Linking, which is the core step. Simply put, it is to smoothly transfer the original class definition information into the JVM running process. This can be further broken down into three steps: 1. Verification, which is an important guarantee for virtual machine security. The JVM needs to verify that the byte information complies with the Java virtual machine specification, otherwise it will be considered a VerifyError, thus preventing If malicious information or non-compliant information harms the operation of the JVM, the verification phase may trigger the loading of more classes. 2. Preparation (Pereparation), create static variables in the class or interface, and initialize the initial values ??of the static variables. But there is a difference between the "initialization" here and the display initialization phase below. The focus is on allocating the required memory space and will not execute further JVM instructions. 3. Resolution. In this step, the symbolic reference in the constant pool will be replaced with a direct reference. In the Java Virtual Machine Specification, the analysis of various aspects such as classes, interfaces, methods and fields is introduced in detail.

The last step is the initialization phase (initialization). This step actually executes the code logic of class initialization, including the action of static field assignment, and the execution of the logic in the static initialization block in the class definition. The compiler performs the compilation phase This part of the logic will be sorted out, and the initialization logic of the parent type will take precedence over the logic of the current type. Let’s talk about the parent delegation model. Simply put, when the loader (Class-Loader) tries to load a certain type, unless the parent class loader cannot find the corresponding type, try to delegate the task to the parent of the current loader. Tools to do it. The purpose of using the delegation model is to avoid repeated loading of Java types.

Common scenarios of custom class loaders

To achieve similar in-process isolation, the class loader is actually used as a different namespace, as well as to provide similar containers and modules transformation effect. For example: 1. Two modules depend on different versions of a certain class library. If they are loaded by different containers, they will not interfere with each other. Masters in this area are Java EE and OSGL, JPMS and other frameworks. 2. The application needs to obtain class definition information from different data sources, such as network data sources instead of the local file system. 3. Or you need to manipulate the bytecode yourself and dynamically modify the generated type.

We can generally simply understand the custom class loading process: 1. Find its binary implementation by specifying the name. This is often the part that the custom class loader will "customize", for example, in a specific data source Get bytecode by name, or modify or generate bytecode. 2. Then, create the Class object and complete the class loading process. The conversion of binary information into class objects usually relies on defineClass. We do not need to implement it ourselves. It is a final method. With the Class object, the subsequent loading process will be completed smoothly.

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