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This article is about how to use Class.forName () of Java. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

Main functions of Class.forName ()

Class.forName (xxx.xx.xx) returns a class

The role of Class.forName (xxx.xx.xx) is to require JVM to find and load the specified class, which means that JVM executes the static code snippet of that class.

Next, we will explain in detail the use of Class.forName () by answering the following three questions.

one。 When do I use Class.forName ()?

I give you a string variable that represents the package name and class name of a class. how do you instantiate it? The first thing that comes to mind is new, but be aware of one thing:

An a = (A) Class.forName ("pacage.A") .newInstance ()

This is the same effect as you An a = new A ();.

Now let's get back to business.

Dynamically load and create Class objects, such as when you want to create objects based on strings entered by the user:

String str = "string entered by the user"; Class t = Class.forName (str); t.newInstance ()

When initializing a class and generating an instance, what is the main difference between the newInstance () method and the new keyword except that one is a method and the other is a keyword? The difference lies in the way objects are created, the former using the class loading mechanism and the latter creating a new class. So why are there two ways to create objects? This mainly takes into account the software design ideas such as scalability, expandability and reusability.

The factory pattern in Java often uses the newInstance () method to create objects, so you can find out why you want to use the factory pattern. For example:

Class c = Class.forName ("Example"); factory = (ExampleInterface) c.newInstance ()

ExampleInterface is the interface of Example, which can be written as follows:

String className = "Example"; class c = Class.forName (className); factory = (ExampleInterface) c.newInstance ()

Further, it can be written as follows:

String className = readfromXMlConfig;// gets the string class c = Class.forName (className) from the xml configuration file; factory = (ExampleInterface) c.newInstance ()

The class name of Example no longer exists in the above code, and its advantage is that no matter how the Example class changes, the above code remains the same, and you can even change the sibling classes Example2, Example3, Example4 of Example. As long as they inherit ExampleInterface

From JVM's point of view

When we use the keyword new to create a class, the class may not be loaded. However, when using the newInstance () method, you must ensure that:

1. This class has been loaded

2. This class is already connected.

What completes the above two steps is the static method forName () of Class, which calls to start the classloader, the loader that loads java API.

Now you can see that newInstance () actually breaks down the new approach into two steps, first calling the Class load method to load a class, and then instantiating it. The benefits of this step-by-step approach are obvious. We can get more flexibility when calling class's static loading method forName, providing a means of decoupling.

II. What's the difference between new and Class.forName ()?

In fact, some of the above has already been mentioned, so let's make a summary here:

First of all, newInstance () is a method and new is a keyword

Second, the use of newInstance () under Class is limited because it generates objects that can only call no-argument constructors, while generating objects using the new keyword does not have this restriction.

In short:

NewInstance (): weakly typed, inefficient, and can only call no-parameter constructions.

New: strongly typed, relatively efficient, and able to call any public construct.

Class.forName ("") returns the class.

Class.forName (") .newInstance () returns object.

three。 Why is Class.forName () useful when loading the database driver package, but not calling newInstance ()?

The method Class.forName () is often used in Java development, especially in database development.

By querying Java Documentation, we will find that the purpose of using the Class.forName () static method is to dynamically load the class.

Usually during the coding process, after the loading is complete, the static method newInstance () under Class is called to instantiate the object for operation. Therefore, it is useless to use Class.forName () to load the class dynamically, the ultimate goal is to instantiate the object.

Friends with database development experience will find out why some of them didn't call the newInstance () method when we loaded the database driver package.

That is, some jdbc connection databases are written in Class.forName (xxx.xx.xx), while others: Class.forName (xxx.xx.xx). NewInstance (), why are they written in these two ways?

As mentioned just now, the role of Class.forName (""); is to require JVM to find and load the specified class. First of all, you must understand that any class in java must be loaded on a virtual machine to run, and the static code is bound to class. Class loading successfully means that your static code has been executed, and you will not walk this static code in the future.

As we said earlier, the function of Class.forName (xxx.xx.xx) is to require JVM to find and load the specified class. If there is a static initializer in the class, JVM must execute the static code snippet of that class.

The JDBC specification explicitly requires that this Driver class must register itself with DriverManager, that is, the code of any JDBC Driver Driver class must be similar to the following:

Public class MyJDBCDriver implements Driver {static {DriverManager.registerDriver (new MyJDBCDriver ());}}

Now that we have registered in the static initializer, we only need Class.forName (XXX.XXX); when using JDBC.

Thank you for reading! This is the end of the article on "how to use Class.forName () of Java". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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