Currently, the message returned from JCDiagnostic.getMessage(Locale) (we access it through Diagnostic.getMessage(Locale)) starts with the absolute path to the file which contains the error and line number where the error occurred. I think that for the users of Diagnostic.getMessage(Locale) it would be more appropriate to get only the "short" error message from this method (like JCDiagnostic.getText()).

Consider the following program:
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import java.io.IOException;
import java.net.URI;
import java.util.Arrays;
import java.util.Collections;
import javax.tools.Diagnostic;
import javax.tools.DiagnosticListener;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.ToolProvider;

public class Main {
    public static void main(String[] args) throws Exception {
        String dest = System.getProperty("java.io.tmpdir");
        class DummyFO extends SimpleJavaFileObject {
            public DummyFO() {
                super(URI.create("nowhere:/Test.java"), JavaFileObject.Kind.SOURCE);
            }
            public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException {
                return "public class Test {sdf}";
            }
        }
        JavaCompiler.CompilationTask task = ToolProvider.getSystemJavaCompiler().getTask(
                null, null, new DL(),
                Arrays.asList("-d", dest),
                null, Collections.singleton(new DummyFO()));
        
        task.call();
    }
    
    private static class DL implements DiagnosticListener {
        
        public void report(Diagnostic diagnostic) {
            System.err.println("reported diagnostic:");
            System.err.println(diagnostic.getMessage(null));
        }
        
    }
}
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Currently, it prints:
reported diagnostic:
nowhere:/Test.java:1: <identifier> expected

But, I think it would be better if it would print:
reported diagnostic:
<identifier> expected