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I first met this problem in my Ideas, I wrote a Class that extends javax.servlet.http.HttpServlet, My Ideas throws an error message reads Error: Could not find or load main class com.bjpowernode.OneServlet, here is the image: enter image description here

It can be seen that Idea didn't show the red wavy line, that shows my codes are fine. I found a solution to this problem from enter link description here, I changed the scope from provided to compile: enter image description here But I actually want to know why and how it works? I compared the difference of Idea compilation instructions under different scope setting，I found that when Idea uses java command to run the .Class, the parameter -classpath of the command of the compile scope has two more paths: D:\apache-tomcat-8.0.50\lib\jsp-api.jar;D:\apache-tomcat-8.0.50\lib\servlet-api.jar That's to say, Idea didn't consider external library paths when run .class under the provided scope, and the super Class HttpServlet is from servlet-api.jar package. Why? To simplify the problem, I created two different classes under two different paths and packages: Class Base and Class Sub, and Sub extends Base. The codes of the Base is here:

package base;
public class Base{
    public static void main(String[] args) {
    
    }
}

The codes of the Sub is here:

package sub;
import base.Base;
public class Sub extends Base{
    public static void main(String[] args) {
        System.out.println("In Sub");
    }
}

The path of Base is ./path1/base/Base.java and the path of Sub is ./path2/sub/Sub.java. I compiled them using these two commands:

javac ./path1/base/Base.java -d ./path1
javac ./path2/sub/Sub.java   -d ./path2  -cp "./path1;./path2"

And compiled successfully. But when I run sub.Sub using the command below:

java sub.Sub -cp "./path1;./path2"

And I got the same error:

Error: Could not find or load main class sub.Sub

Ive tried multiple variations of this, but none of them seem to work. Any ideas? Although I solved the problem of idea reporting errors, I still could not understand the principle behind? I hope this question can help me to figure it out. My jdk version is 1.8. Thanks in advance.

kuze mhw
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  • _Idea didn't consider external library paths when run .class under the provided scope_ because the "provided" scope means that this library is provided by the runtime environment and IDEA should not place it into the classpath itself. – Thomas Kläger Feb 18 '21 at 06:15
  • Thank you for answering my questions. I understande Idea 's setting now. And now I figure it out that why the command java sub.Sub -cp "./path1;../path2; didn't work. The key is the parameter sequence, if -cp parameter is set before sub.Sub just like java -cp "./path1;./path2" sub.Sub it works!!!! I thought the order of the parameters of the command is free, but obviously the java command is not. To my surprise, the -cp of the javac command can be placed anywhere. That means both of the following commands about javac work: – kuze mhw Feb 19 '21 at 04:09
  • javac -cp "./path1;.;" ./path2/sub/Sub.java javac ./path2/sub/Sub.java -cp "./path1;.;" – kuze mhw Feb 19 '21 at 04:18
  • Yes, for the java command the order matters: the JVM options first (including the classpath), then the class name (sub.Sub), then the arguments for your `main()` method. Otherwise the java command could not distinguish between the options for the JVM and the arguments for your `main()` method. – Thomas Kläger Feb 19 '21 at 06:29
  • For the javac things are a little different: the options for the JVM are prefixed with `-J`, all other arguments are for the compiler. – Thomas Kläger Feb 19 '21 at 06:30
  • Thank you again for helping me answer the question. Now I fully understand. – kuze mhw Feb 19 '21 at 14:45

0 Answers0