Let you understand the JAVA reflection mechanism (summary sharing)-javaTutorial-php.cn

This article brings you relevant knowledge about java, which mainly introduces issues related to the reflection mechanism, including what reflection is, what reflection can do, reflection-related APIs, etc., I hope everyone has to help.

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1. What is reflection?

A very important concept in Java development is the Java reflection mechanism, which is also one of the important features of Java.

The concept of reflection was first proposed by Smith in 1982. It mainly refers to the ability of a program to access, detect and modify its own state or behavior. Through reflection, private methods and private properties can be called. Some frameworks also use the principle of reflection.

Reflection (reflection) is regarded as the key to dynamic language. The reflection mechanism allows the program to obtain the internal information of any
class during execution with the help of the Reflection API, and can directly operate it Internal properties and methods of any object.

A class has multiple components, such as member variables, methods, constructors, etc. Reflection is to load the class and dissect the various components of the class.

2. What can reflection do?

Java's reflection mechanism knows the basic structure of the class. This ability to detect the structure of the Java class is called the "self-examination" of the Java class. For example, in eclipse, with one click, the compilation tool will automatically list all the methods and properties that can be used by the object for the user to choose. This uses the principle of Java reflection to detect and self-examine the objects we create.

Reflection can do:

Determine the class to which any object belongs at run time;
Construct an object of any class at run time;
Determine the member variables and methods of any class at run time;
Obtain generic information at run time;
Call the member variables and methods of any object at run time ;
Annotations are processed at runtime;
Generate dynamic proxy;

3. Reflection related API

##java .lang.Class:Source of reflection
:Method
: Attribute
: Constructor…
4. Understanding of Class class

(1) Introduction:

After the program passes the javac.exe command, it will generate one or more bytecode files (ending with .class). Then we use the java.exe command to interpret and run a certain bytecode file. Equivalent to loading a certain bytecode file into memory. This process is called

class loading

. A class loaded into memory is called a runtime class, and this runtime class is used as an instance of Class. In other words, an instance of Class corresponds to a runtime class.

The runtime classes loaded into memory will be cached for a certain period of time. Within this time, we can obtain this runtime class in different ways

(2). Class loading process:

Loading: In our new object or use

Class.forName("Package name. Class")

When the class loader (ClassLoader) will load the class into memory and create a Class object

How to get the Class object?

class
getClass()
("Package name.Class");

Link:

The work done by the link is mainly to verify whether the bytecode is legal, allocate memory space for static and initialize it (not real initialization, just Give the corresponding type of variable a default value, such as int to 0, double to 0.0)

Unload:

Unload from memory ( We don’t need to care about when to uninstall, it will be done by the JVM)

(3) Class loader

Class (CLASS) can only run after it is loaded into the JVM . When running the specified program, the JVM will load the compiled .class file into memory according to requirements and certain rules, and organize it into a complete Java application. This loading process is completed by the class loader, specifically, it is implemented by ClassLoader and its subclasses. The class loader itself is also a class. Its essence is to read the class file from the hard disk into the memory!

Classification of class loaders:

BootStrap: is mainly responsible for loading the core class library (java.lang.*, etc.) and constructing ExtClassLoader and APPClassLoader;
ExtClassLoader: Mainly responsible for loading some extended jar packages in the jre/lib/ext directory;
AppClassLoader: Mainly responsible for Load the main function class of the application (the java file written by yourself is loaded by this class loader);

System.out.println("app:" + System.getProperty("java.class.path"));
System.out.println("ext:" + System.getProperty("java.ext.dirs"));
System.out.println("----bootstrap---");
String[] str = System.getProperty("sun.boot.class.path").split(";");

for (String s : str) {
    System.out.println(s);
}

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Parental delegation (dispatch) mechanism:

When a file like Hello.class is to be loaded. Regardless of our custom class loader, we will first check whether it has been loaded in AppClassLoader. If so, there is no need to load it again. If not, the parent loader will be obtained, and then the loadClass method of the parent loader will be called. In the same way, the parent class will first check whether it has been loaded, and if not, go up. Pay attention to this recursive process. Until it reaches the Bootstrap classLoader, it is checking whether it has been loaded and will not choose to load it by itself. Until BootstrapClassLoader, there is no parent loader. At this time, I start to consider whether I can load it. If I can't load it, I will sink to the child loader to load it, all the way to the bottom layer. If no loader can load it, it will Throws ClassNotFoundException. So does anyone have the following questions?

Why should we design this mechanism?

One advantage of this design is that if someone wants to replace the system-level class: String.java. Tamper with its implementation. Under this mechanism, these system classes have been loaded by Bootstrap classLoader (why? Because when a class needs to be loaded, the first thing to try to load is BootstrapClassLoader), so other class loaders have not Loading again when the opportunity arises prevents the implantation of dangerous code to a certain extent.

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