Test if the returned string has java class

Question

I have a method that generates java class and writes to a .java file. How can write a unit test on this method to make sure the format of the string this is writing to the file is in standard java class format.

Ex: I should check if it has a package declaration Should check if package is before class declaration open and close braces etc...

Why not just try to compile the source code and then make assertions about the compiled class? — Daniel Pryden, Apr 26 '16 at 19:47

score 0 · Accepted Answer · edited May 23 '17 at 11:52

Here's a method which can be used to find the compilation errors in a Java files, if there are no errors generated then it's a perfectly valid Java Class.

import java.io.File;
import java.io.IOException;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import javax.tools.Diagnostic;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;

public class TestMain {

    public static void main(String[] args) {
        List<File> sourceList = Arrays.asList(Paths.get("MyJavaClass.java").toFile());
        List<String> errorList = new TestMain().compile(sourceList);
        if(errorList.size() == 0) {
            System.out.println("No error found, perfectly valid java class");
        } else {
            errorList.forEach(System.out::println);
        }
    }

    public List<String> compile (List<File> javaFileList) {
        System.out.println("Started compilation");
        List<String> errorList = new ArrayList<String>();
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(
                diagnostics, null, null);

        Iterable<? extends JavaFileObject> compilationUnits = fileManager
                .getJavaFileObjectsFromFiles(javaFileList);
        compiler.getTask(null, fileManager, diagnostics, null, null, compilationUnits)
                .call();

        for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics
                .getDiagnostics()) {
            String diagnosticMessage = String.format("Error on line %d in %s%n",
                    diagnostic.getLineNumber(), diagnostic.getSource().toUri() + " : \n\t" + diagnostic.getMessage(null));

            /*Following gives out of box good message, but I used above to show the custom use of diagnostic
             * String diagnosticMessage = diagnostic.toString();*/

            errorList.add(diagnosticMessage);
        }
        try {
            fileManager.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return errorList;
    }
}

Taken from Best way to process/handle Error stream messages

score 0 · Answer 2 · edited May 23 '17 at 12:07

If you just want to check if it is a valid Java class (that can be compiled), you can try.

 try {
            Class<?> clazz = MyTest.class.getClassLoader().loadClass("TargetClass");
            clazz.newInstance();
            } catch (Throwable e) { // Not a good idea to catch error, for test purpose only.
                if(e instanceof Error && e.getMessage().contains("Unresolved compilation problems")){
                    Assert.fail("Invalid class");
                }
            }

You might have to call "javac" process (see how to compile & run java program in another java program?) to make sure TargetClass.class is available before running this test check.

Test if the returned string has java class

2 Answers2

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