Project packages cannot import Java compiler API: ClassNotFoundException / NoClassDefFoundError

Question

I use Java compiler class inside my Spring Boot application as follows:

 String classStr = "\n" +
        "import java.util.List;\n" +
        "import java.util.Map;\n" +
        "import java.util.Arrays;\n"+
        "public class TestClass {\n" + String.join("\n",methods) + "}";

File root = new File("/src/main/resources/");
File sourceFile = new File(root, "TestClass.java");
sourceFile.getParentFile().mkdirs();
Files.write(sourceFile.toPath(), classStr.getBytes(StandardCharsets.UTF_8));

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
compiler.run(null, null, null,sourceFile.getPath());

URLClassLoader classLoader = URLClassLoader.newInstance(new URL[]{root.toURI().toURL()});
cls = Class.forName("TestClass", true, classLoader);

It is working as expected and cls methods can be invoked without any problem.

Then I try to import and use a class inside my project src as follows:

String classStr = "\n" +
        "import java.util.List;\n" +
        "import java.util.Map;\n" +
        "import java.util.Arrays;\n"+
        "import com.nilanka.compilertest.Utility;\n"+
        "public class TestClass {\n" + String.join("\n",methods) + "}";

It gave me the error at the start:

Caused by: java.lang.ClassNotFoundException: Utility
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)

Then I tried:

compiler.run(null, null, null,
  -cp",Utility.class.getProtectionDomain().getCodeSource().getLocation().getPath(),sourceFile.getPath());

then Application started without any error.

But when I try to execute a method from new class:

java.lang.NoClassDefFoundError: com/nilanka/compilertest/Utility

How can I resolve this problem?

I need a way to make the imports available in both compiling and runtime for the class.

Does this answer your question? What causes and what are the differences between NoClassDefFoundError and ClassNotFoundException? — vaibhavsahu
– vaibhavsahu, Commented Aug 27, 2021 at 4:32

java-coding-prodigy · Accepted Answer · 2021-08-27 04:03:20Z

In the snippets you shared, there is no declaration of class and methods, you might not have defined it at all and. You can declare a class with the name Cls It would go like this

public class Cls{
      public static void main(String[]args){
           String classStr = "\n" +
            "import java.util.List;\n" +
            "import java.util.Map;\n" +
            "import java.util.Arrays;\n"+
            "public class TestClass {\n" + String.join("\n",methods) + "}";


           File root = new File("/src/main/resources/");
           File sourceFile = new File(root, "TestClass.java");
           sourceFile.getParentFile().mkdirs();
           Files.write(sourceFile.toPath(), 
               classStr.getBytes(StandardCharsets.UTF_8));

           JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
           compiler.run(null, null, null,sourceFile.getPath());

           URLClassLoader classLoader = URLClassLoader.newInstance(new URL[]{                       
               root.toURI().toURL()});
           cls = Class.forName("TestClass", true, classLoader);
     }
}

Try running this. It might work :)

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