Question

I believe that we are all know that setUp (@Before) will execute before any test method and tearDown(@After) will execute after test method.

Also we know that Junit will create one instance of Test per test method.

my question is that can we just move setUp method content to class Constructor and remove setUp method? is there any specific reason to keep setUp method?

Answer 1

This (old) JUnit best practices article puts it like this:

Do not use the test-case constructor to set up a test case

Setting up a test case in the
constructor is not a good idea.
Consider:

public class SomeTest extends TestCase
   public SomeTest (String testName) {
      super (testName);
      // Perform test set-up
   }
}

Imagine that while performing the
setup, the setup code throws an
IllegalStateException. In response,
JUnit would throw an
AssertionFailedError, indicating that
the test case could not be
instantiated. Here is an example of
the resulting stack trace:

junit.framework.AssertionFailedError: Cannot instantiate test case: test1   
    at junit.framework.Assert.fail(Assert.java:143)
    at junit.framework.TestSuite.runTest(TestSuite.java:178)
    at junit.framework.TestCase.runBare(TestCase.java:129)
    at junit.framework.TestResult.protect(TestResult.java:100)
    at junit.framework.TestResult.runProtected(TestResult.java:117)
    at junit.framework.TestResult.run(TestResult.java:103)
    at junit.framework.TestCase.run(TestCase.java:120)
    at junit.framework.TestSuite.run(TestSuite.java, Compiled Code)
    at junit.ui.TestRunner2.run(TestRunner.java:429)

This stack trace proves rather
uninformative; it only indicates that
the test case could not be
instantiated. It doesn’t detail the
original error’s location or place of
origin. This lack of information makes
it hard to deduce the exception’s
underlying cause.

Instead of setting up the data in the
constructor, perform test setup by
overriding setUp(). Any exception
thrown within setUp() is reported
correctly. Compare this stack trace
with the previous example:

java.lang.IllegalStateException: Oops
    at bp.DTC.setUp(DTC.java:34) 
    at junit.framework.TestCase.runBare(TestCase.java:127)
    at junit.framework.TestResult.protect(TestResult.java:100)
    at junit.framework.TestResult.runProtected(TestResult.java:117)
    at junit.framework.TestResult.run(TestResult.java:103)
    ...

This stack trace is much more
informative; it shows which exception
was thrown (IllegalStateException) and
from where. That makes it far easier
to explain the test setup’s failure.