How to solve the problem that the main method is not defined in Java?

As an object-oriented programming language, Java requires an entry point when executing a program. This entry point is the main method. But sometimes we encounter a situation where the main method is not defined, so how to solve it?

1. The main method is not defined

The main method is not defined. This may be because the code has been modified or other people's code has been imported. At this time we can use reflection to deal with it.

1. Use reflection.

Reflection is a basic mechanism in the Java language that can load, operate and query classes and interfaces while the program is running. With reflection, you don't need to know all classes and interfaces at compile time. The Java Virtual Machine (JVM) only needs to load classes and interfaces when the program is running. Therefore, the reflection mechanism allows Java programs to dynamically load and operate classes and interfaces at runtime.

The sample code is as follows:

public class NoMainClass {

public void sayHello() {
    System.out.println("Hello");
}

}
public class Main {

public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
    Class<?> clazz = Class.forName("NoMainClass");
    Object o = clazz.newInstance();
    Method method = clazz.getMethod("sayHello");
    method.invoke(o);
}

}

This In the code snippet, the program dynamically loads a class at runtime, creates an instance of the class through the newInstance() method, and dynamically obtains the method of this instance and calls it. This is how to use the reflection mechanism to dynamically load a class without a main method and execute it.

2. Use Java 9 module system.

When we need to use the Java 9 module system to solve the problem of not defining the main method, we need to define a main class in the module description file. For example, we can define the following content in the module-info.java file:

module my.module {

requires other.module;
exports com.example;
provides service with com.example.MyServiceImpl;
main-class com.example.MyMain;

}

In this module description file, we pass Specify main-class to specify the entry point of the program. This allows the Java virtual machine to find the class that defines the main method and execute it.

2. Summary

Some Java programmers may encounter the problem of not defining the main method at work or study. At this time, we can use reflection and Java 9 module system and other mechanisms to solve it. . However, it should be noted that this approach may have security risks, so we should avoid using this method as much as possible. When writing code, you should follow the Java language specifications and define a clear program entry point.

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