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Editor to share with you the example analysis of the class loading mechanism in JVM, I believe that most people do not know much about it, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!

Loading and unloading of classes

JVM is a kind of virtual machine, its instruction set language is bytecode, and the file composed of bytecode is class file. Normally, the Java file we write needs to be compiled into a class file before it can be run by JVM. It can be said that C language code-- > binaries-- > computer hardware is equivalent to Java code-- > bytecode file-- > JVM. The process of JVM reading the specified class file into memory and running the Java program in the class file is called class loading; conversely, the process of removing the runtime data of a class file from the JVM is called class unloading.

The runtime data of class files is C++ objects, also known as kclass objects, which are placed in PermGen before JDK7 and in Metaspace after JDK8.

The life cycle of a class

From loading by virtual machine to unloading out of memory, the whole life cycle of Java class includes seven stages: Loading, Verification, Preparation, Resolution, Initialization, Using and Unloading, in which verification, preparation and parsing are collectively called Linking phase.

The timing of class loading the virtual machine specification does not strictly specify the timing of class loading, which is related to the specific JVM virtual machine. The best time for class loading is when parsing the constant pool symbols in the Java bytecode class file. Class.forName (), ClassLoader.loadClass (), reflection API, and JNI_FindClass can all trigger class loading, and class loading will also be triggered when Hot JVM itself starts.

By adding-verbose:class to the JVM parameter, you can print the process of class loading when the application starts, as shown in the following figure: during the initialization phase, the JVM virtual machine gives five situations in which classes must be "initialized"

When instantiating an object using the new keyword, when reading or setting a class's static field, when calling a class's static method

When a reflection call is made to a class using the method of the java.lang.reflect package, if the class has not been initialized, its initialization should be triggered first.

When initializing a class, if you find that its parent class has not been initialized, initialize its parent class first

When the virtual machine starts, the user needs to specify an executing main class (the class that contains the main method), and the virtual machine initializes the main class first.

After JDK1.7, when dynamic languages are supported, initialization is also required if the end result of an java.lang.invoke.MethodHandle instance is to perform the operation in the first case. The unloading time of a class is related to the garbage collection algorithm used. In CMS, there are two ways to unload unnecessary classes, one is to trigger FGC when the Metaspace is full, and the other is to use a method similar to the CMS concurrent collection algorithm, but the threshold for metaspace and the threshold for triggering CMS concurrent collection are independent. For more details, you can refer to the previous article: CMS Learning Notes. Here, we just need to remember that the unloading of a class in JVM meets the following three conditions:

All instance objects of this class have been recycled

The class loader object of this class has been reclaimed

The corresponding java.lang.Class object of this class is not referenced anywhere, and its methods cannot be accessed anywhere through reflection.

The role of the classloader the loading of the class needs to be done by the classloader, but the role of the classloader in JVM is more than that. In JVM, the uniqueness of a class can only be determined by the class itself and class loading, and each class loader has a separate namespace.

Different class loaders, even if it is the same class bytecode file, do not have the same class object in JVM. The following code shows this conclusion:

As you can see, the jvm.ClassLoaderTest class loaded with the custom class loader (myLoader) in the code is not the same class as the class loaded through the application class loader. To sum up, the role of class loaders in JVM is as follows:

Load the bytecode file of the class into memory from outside JVM

Determine the uniqueness of a class

Provide isolation features to facilitate middleware developers, such as Tomcat

The above is all the content of the article "sample Analysis of Class loading Mechanism in JVM". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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