Java 中 `Class.forName()` 和 `Class.forName().newInstance()` 之間的主要差異是什麼？

Understanding the Distinction Between "Class.forName()" and "Class.forName().newInstance()"

In Java, both Class.forName() and Class.forName().newInstance() play crucial roles in dynamic class loading. However, they serve distinct purposes.

Class.forName()

The Class.forName() method takes a fully qualified class name as a string and returns the Class object associated with that class. It does this by using the class loader to locate the class file and read its contents. Class.forName() does not instantiate the class. It simply provides access to its metadata and information.

Class.forName().newInstance()

The Class.forName().newInstance() method, on the other hand, creates a new instance of the class represented by the Class object returned by Class.forName(). It does this by invoking the class's default constructor.

Example:

Let's consider an example to illustrate the difference:

package test;

public class Demo {

    public Demo() {
        System.out.println("Hi!");
    }

    public static void main(String[] args) throws Exception {
        Class clazz = Class.forName("test.Demo");
        Demo demo = (Demo) clazz.newInstance();
    }
}

In this example:

• Class clazz = Class.forName("test.Demo"): This line loads the Demo class using Class.forName(). clazz now holds the Class object for the Demo class.
• Demo demo = (Demo) clazz.newInstance(): This line creates a new instance of the Demo class using Class.forName().newInstance(). The (Demo) cast is necessary because clazz.newInstance() returns an Object.

Key Difference:

The key difference between Class.forName() and Class.forName().newInstance() is that Class.forName() only loads the class, while Class.forName().newInstance() not only loads the class but also creates an instance of that class.

Dynamic Class Loading Use Cases:

Class.forName() and Class.forName().newInstance() are commonly used in scenarios where dynamic class loading is required. Examples include:

• JDBC drivers loading at runtime based on database requirements.
• EJB and Servlet containers dynamically loading and creating components.
• Plugins or extensions that are loaded on demand.

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