测试一个简单的hello world方法

Question

I have worked with junit test integration tests and controller tests in spring and usually we test the output of a method but when i tried to test a simple hello world in main method i had no idea how to go about it so will like to get any idea on what do write

public class App 
{
    public static void main( String[] args )
    {
        System.out.println( "Hello World!" );
    }
}

This is the simple java class any idea how i can test it I tried to write something like this

public void mainMethodTest() throws Exception{

        System.out.println("hello world");
        String[] args = null;


        Assert.assertEquals(System.out.println("hello world"),App.main(args));
    }

Answer 1

You could assign to the System.out variable a ByteArrayOutputStream object which you store the reference in a variable.
Then invoke your main() method and assert that the String content of the ByteArrayOutputStream object contains the expected String :

@Test
public void main() throws Exception{                
    PrintStream originalOut = System.out; // to have a way to undo the binding with your `ByteArrayOutputStream` 
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    System.setOut(new PrintStream(bos));
    // action
    App.main(null);
    // assertion
    Assert.assertEquals("hello world", bos.toString());
    // undo the binding in System
    System.setOut(originalOut);
}

Why does it work ?

bos.toString() returns the "Hello World!" String passed in the method under test:

System.out.println( "Hello World!" );

as after setting System.out in this way : System.setOut(new PrintStream(bos)); , the out variable refers to a PrintStream object that decorates the ByteArrayOutputStream object referenced by the bos variable. So any System.out invocations will write byte s in the ByteArrayOutputStream object.

Answer 2

You can change your class this way

import java.io.PrintStream;

public class TestHelloWorld {

    public final static void main(String[] args) {
        doPrint(System.out);
    }

    static void doPrint(PrintStream ps) {
        ps.println("Hello World");
    }
}

and test the doPrint function by providing your own PrintStream you create around a ByteArrayOutputStream :

public void mainMethodTest() throws Exception{
    ByteArrayOutputStream data = new ByteArrayOutputStream();
    PrintStream ps = new PrintStream(data, true, "UTF-8");
    TestHelloWorld.doPrint(ps);
    ps.flush();
    Assert.assertEquals("Hello World") + System.getProperty("line.separator"), new String(data, "UTF-8"));
}

Another solution is to replace the system's PrintStream by your own:

System.setOut(new PrintStream(data, true, "UTF-8"));

but that's quite ugly and I try to avoid that. Above solution is more clear, easier to maintenance and you can be sure that no other part of a larger application is printing something to STDOUT while you do your test, leading to a failure of it.

Answer 3

You can run Junit from a main method if thats what you mean.

public static void main( String[] args )
{
    JUnitCore junit = new JUnitCore(); 
    Result result = null;
    try {        
         result = junit.run(MyTestClass.class);
    }
    catch(Exception e)
    {
        e.printStackTrace();
    }
    int passed = result.getRunCount()-result.getFailureCount();
}

public class MyTestClass{
    @Test
    public void testAllBrowsers(){
         //test code and asserts
    }
}