JVM class loading subsystem

1. Class loader and class loading process

Let's look at a picture first , Bytecode files go through the process of class loading system , First of all, we need to be clear about , What is the role of a class loader ？

It mainly plays a role of transmission , Equivalent to a courier , The later process can be regarded as an assembly line , The corresponding documents will be verified , Judge whether it is legal .

Class loading process

load

The loading here is only the initial process of the whole loading , This is something to be aware of , Loading is mainly divided into three steps .

1. Get the binary byte stream of the corresponding class through the fully qualified class name of the class .

2. Convert the static structure represented by byte stream into the runtime structure of method area .

3. Generate the corresponding... In memory java.lang.Class object .

link

The link is broken down into three steps , These three steps are very important .

verification

Someone may change it maliciously class file , Achieve some kind of attack , So we should verify it first .

Get ready

• Assign a value of zero to the class variable
• Add final The variables of are defined at compile time , So no assignment
• At this time, the object has not been created , So instance variables will not be assigned

analysis
Convert symbolic references to direct references , The specific details of bytecode will be discussed again .

initialization

First of all, understand clint Methods and init Method , The former is assigned by all class variables , Methods for merging static statements , The latter has several constructors , There will be several corresponding init Method .

clint The method is to use static Embellished sentences , Will be added to it , If a class has a parent , Then the parent class will be executed first clinit Method .

init Methods contain constructors , Explicitly initialize , Such as direct assignment , Code block , Instantiation is included .

The whole initialization phase is execution clinit Methodical , And it's executing clinit Method will lock , Ensure data security .

2. Classloader classification

Class loaders are mainly divided into two categories , One is C language-written , go by the name of Boot class loader , Others can be counted as custom loaders , The details are as follows .

The top layer is the most special , The rest is made up of Java The realization of language .

You can run the following code , Try to get the next class loader .

public class Test {
    public static void main(String[] args) {
        // Get system classloader 
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader);//sun.misc.Launcher$ [email protected]

        // Get extended class loader  
        ClassLoader extClassLoafer = systemClassLoader.getParent();
        System.out.println(extClassLoafer);//sun.misc.Launcher$  [email protected]

        // User defined class loader   
        ClassLoader classLoader = Test.class.getClassLoader();
	
        System.out.println(classLoader);//sun.misc.Launcher$  [email protected]

        // Try to get the boot class loader   
        ClassLoader bootstrapClassloader = extClassLoafer.getParent();
        System.out.println(bootstrapClassloader);//null
    }
}

You can see , We found that , The custom class loader is the system class loader , The upper level of the system class loader is the boot class loader , The upper level of the boot class loader is null, the reason being that C Written language , We can't get .

3. ClassLoader Instructions for use

Bootstrap classLoader

Boot class loader , Also known as boot class loader , The role of the most superior , use C Language writing , Used to load some core classes , Such as JAVA_HOME/jre/lib/rt.jar、resource.jar, Provide JVM The class needed by itself .

Extension classLoader

Extend the classloader , Derive from ClassLoader class , from Java Language writing , Load content other than the core package , Such as user built Jar bag jre/lib/ext Under the table of contents , It will also be loaded by the extension class loader .

AppClassLoader

system class loader , Generally speaking, the classes we define are loaded by it , It is also the default loader in the program .

In some cases , We really need custom loaders , Such as isolation loading class , Modify the loading method of avocado , Prevent source code leakage , The details will be discussed later .

4. Talk about the parental delegation mechanism

I was asked about this by the interviewer , In fact, I don't think this translation is accurate , It should be called parent agent , Parents don't mean parents , It refers to the higher level of the loader .

The flow chart is shown below .

The specific process is divided into three steps .
1. When a class receives a load request , It doesn't load directly , Instead, it is executed by its parent class loader
2. If there is a parent loader , Just entrust upward , Until you know your superiors
3. If the top-level loader can finish loading, it is certainly best , If not , Then it is handled by the subclass loader .

You can imagine a delicious meal , Although children want to eat , But first push it to Dad , Dad wants to , Grandpa hasn't moved his chopsticks yet , Wait until grandpa eats , Grandpa said , This thing is too harsh , You eat it , Dad ate it first ......

Let's give you an example , Define a java.lang Under bag Hello class

But the operation directly reported an error , Security exception , Forbidden package name java.lang

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.lang
	at java.lang.ClassLoader.preDefineClass(ClassLoader.java:662)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:761)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)

Because the classes under this package are managed by the boot class loader , If it can be defined like this , So if you write a dead circle , Or delayed procedures , It may cause the loader to run abnormally , This is also a protective measure .

5. other

Interview questions ： If you judge two class Whether the object is the same class ？

First, the complete package names of the two classes should be consistent , And their loaders should also be consistent , Only when you answer the second point can you distinguish yourself from others .

Active and passive use of classes

Active reference will lead to class initialization , You can recall the first section , That is, whether it will be implemented clinit Method .

There are seven

• Create an instance of a class
• Accessing a class / Static variables of the interface , Or assign a value to the static variable
• Calling static methods of a class
• Reflection get class
• JVM The class that is indicated to be started at startup ( Remember this first I haven't met )
• JDK 7 Start with dynamic language support ：
  java.lang.invoke.MethodHandle The parsing result of the instance
  REF_getStatic、REF_putStatic、REF_invokeStatic The class corresponding to the handle is not initialized ( Understanding can )

The last two are rarely encountered , Understanding can .

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