JUnit test for System out println

Question

I need to write JUnit tests for an old application that s poorly designed and is writing a lot of error messages to standard output  When the getResponse String request  method behaves correctly it returns a XML response    BeforeClass public static void setUpClass   throws Exception       Properties queries   loadPropertiesFile  requests properties        Properties responses   loadPropertiesFile  responses properties        instance   new ResponseGenerator queries  responses       Test public void testGetResponse         String request     lt some gt request lt  some gt        String expResult     lt some gt response lt  some gt        String result   instance getResponse request       assertEquals expResult  result       But when it gets malformed XML or does not understand the request it returns null and writes some  stuff to standard output   Is there any way to assert console output in JUnit  To catch cases like   System out println  match found      strExpr   System out println  xml not well formed      e getMessage

User · Answer

If the function is printing to System out  you can capture that output by using the System setOut method to change System out to go to a PrintStream provided by you  If you create a PrintStream connected to a ByteArrayOutputStream  then you can capture the output as a String     Create a stream to hold the output ByteArrayOutputStream baos   new ByteArrayOutputStream    PrintStream ps   new PrintStream baos      IMPORTANT  Save the old System out  PrintStream old   System out     Tell Java to use your special stream System setOut ps      Print some output  goes to your special stream System out println  quot Foofoofoo  quot       Put things back System out flush    System setOut old      Show what happened System out println  quot Here   quot    baos toString

User · Answer

If you were using Spring Boot  you mentioned that you re working with an old application  so you probably aren t but it might be of use to others   then you could use org springframework boot test rule OutputCapture in the following manner    Rule public OutputCapture outputCapture   new OutputCapture      Test public void out         System out print  hello        assertEquals outputCapture toString     hello

User · Answer

You cannot directly print by using system out println or using logger api while using JUnit  But if you want to check any values then you simply can use   Assert assertEquals  value   str     It will throw below assertion error   java lang AssertionError  expected  21 92  but found  value    Your value should be 21 92  Now if you will test using this value like below your test case will pass    Assert assertEquals 21 92  str

User · Answer

dfa answer is great  so I took it a step farther to make it possible to test blocks of ouput    First I created TestHelper with a method captureOutput that accepts the annoymous class CaptureTest  The captureOutput method does the work of setting and tearing down the output streams  When the implementation of CaptureOutput s test method is called  it has access to the output generate for the test block   Source for TestHelper   public class TestHelper        public static void captureOutput  CaptureTest test   throws Exception           ByteArrayOutputStream outContent   new ByteArrayOutputStream            ByteArrayOutputStream errContent   new ByteArrayOutputStream             System setOut new PrintStream outContent            System setErr new PrintStream errContent             test test  outContent  errContent             System setOut new PrintStream new FileOutputStream FileDescriptor out             System setErr new PrintStream new FileOutputStream FileDescriptor out               abstract class CaptureTest       public abstract void test  ByteArrayOutputStream outContent  ByteArrayOutputStream errContent   throws Exception      Note that TestHelper and CaptureTest are defined in the same file   Then in your test  you can import the static captureOutput  Here is an example using JUnit      imports for junit import static package to TestHelper     public class SimpleTest         Test     public void testOutput   throws Exception            captureOutput  new CaptureTest                  Override             public void test ByteArrayOutputStream outContent  ByteArrayOutputStream errContent  throws Exception                       code that writes to System out                  assertEquals   the expected output n   outContent toString

User · Answer

using ByteArrayOutputStream and System setXXX is simple  private final ByteArrayOutputStream outContent   new ByteArrayOutputStream    private final ByteArrayOutputStream errContent   new ByteArrayOutputStream    private final PrintStream originalOut   System out  private final PrintStream originalErr   System err    Before public void setUpStreams         System setOut new PrintStream outContent        System setErr new PrintStream errContent        After public void restoreStreams         System setOut originalOut       System setErr originalErr      sample test cases   Test public void out         System out print  quot hello quot        assertEquals  quot hello quot   outContent toString         Test public void err         System err print  quot hello again quot        assertEquals  quot hello again quot   errContent toString        I used this code to test the command line option  asserting that -version outputs the version string  etc etc  Edit  Prior versions of this answer called System setOut null  after the tests  This is the cause of NullPointerExceptions commenters refer to

User · Answer

You can set the System out print stream via setOut    and for in and err   Can you redirect this to a print stream that records to a string  and then inspect that   That would appear to be the simplest mechanism    I would advocate  at some stage  convert the app to some logging framework - but I suspect you already are aware of this

User · Answer

for out   Test void it prints out          PrintStream save out System out final ByteArrayOutputStream out   new ByteArrayOutputStream   System setOut new PrintStream out         System out println  Hello World         assertEquals  Hello World  r n   out toString          System setOut save out       for err    Test void it prints err          PrintStream save err System err final ByteArrayOutputStream err  new ByteArrayOutputStream   System setErr new PrintStream err         System err println  Hello World         assertEquals  Hello World  r n   err toString          System setErr save err

User · Answer

Slightly off topic  but in case some people  like me  when I first found this thread  might be interested in capturing log output via SLF4J  commons-testing s JUnit  Rule might help   public class FooTest        Rule     public final ExpectedLogs logs   new ExpectedLogs              captureFor Foo class  LogLevel WARN                 Test     public void barShouldLogWarning             assertThat logs isEmpty    is true       Nothing captured yet              Logic using the class you are capturing logs for          Foo foo   new Foo            assertThat foo bar    is not nullValue                   Assert content of the captured logs          assertThat logs isEmpty    is false            assertThat logs contains  Your warning message here    is true              Disclaimer     I developed this library since I could not find any suitable solution for my own needs  Only bindings for log4j  log4j2 and logback are available at the moment  but I am happy to add more

User · Answer

Full JUnit 5 example to test System out  replace the when part    package learning   import static org assertj core api BDDAssertions then   import java io ByteArrayOutputStream  import java io PrintStream  import org junit jupiter api AfterEach  import org junit jupiter api BeforeEach  import org junit jupiter api Test   class SystemOutLT        private PrintStream originalSystemOut      private ByteArrayOutputStream systemOutContent        BeforeEach     void redirectSystemOutStream              originalSystemOut   System out              given         systemOutContent   new ByteArrayOutputStream            System setOut new PrintStream systemOutContent                AfterEach     void restoreSystemOutStream             System setOut originalSystemOut               Test     void shouldPrintToSystemOut                 when         System out println  example             then systemOutContent toString    containsIgnoringCase  example

User · Answer

You don t want to redirect the system out stream because that redirects for the ENTIRE JVM  Anything else running on the JVM can get messed up  There are better ways to test input output  Look into stubs mocks

User · Answer

Based on  dfa s answer and another answer that shows how to test System in  I would like to share my solution to give an input to a program and test its output   As a reference  I use JUnit 4 12   Let s say we have this program that simply replicates input to output   import java util Scanner   public class SimpleProgram       public static void main String   args            Scanner scanner   new Scanner System in           System out print scanner next             scanner close              To test it  we can use the following class   import static org junit Assert     import java io     import org junit     public class SimpleProgramTest       private final InputStream systemIn   System in      private final PrintStream systemOut   System out       private ByteArrayInputStream testIn      private ByteArrayOutputStream testOut        Before     public void setUpOutput             testOut   new ByteArrayOutputStream            System setOut new PrintStream testOut               private void provideInput String data            testIn   new ByteArrayInputStream data getBytes             System setIn testIn              private String getOutput             return testOut toString                After     public void restoreSystemInputOutput             System setIn systemIn           System setOut systemOut               Test     public void testCase1             final String testString    Hello            provideInput testString            SimpleProgram main new String 0             assertEquals testString  getOutput               I won t explain much  because I believe the code is readable and I cited my sources   When JUnit runs testCase1    it is going to call the helper methods in the order they appear    setUpOutput    because of the  Before annotation provideInput String data   called from testCase1   getOutput    called from testCase1   restoreSystemInputOutput    because of the  After annotation   I didn t test System err because I didn t need it  but it should be easy to implement  similar to testing System out

User · Answer

I know this is an old thread  but there is a nice library to do this   System Rules  Example from the docs   public void MyTest        Rule     public final SystemOutRule systemOutRule   new SystemOutRule   enableLog          Test     public void overrideProperty             System out print  hello world            assertEquals  hello world   systemOutRule getLog               It will also allow you to trap System exit -1  and other things that a command line tool would need to be tested for

User · Answer

Instead of redirecting System out  I would refactor the class that uses System out println   by passing a PrintStream as a collaborator and then using System out in production and a Test Spy in the test   That is  use Dependency Injection to eliminate the direct use of the standard output stream    In Production  ConsoleWriter writer   new ConsoleWriter System out      In the Test  ByteArrayOutputStream outSpy   new ByteArrayOutputStream    ConsoleWriter writer   new ConsoleWriter new PrintStream outSpy    writer printSomething    assertThat outSpy toString    is  expected output       Discussion  This way the class under test becomes testable by a simple refactoring  without having the need for indirect redirection of the standard output or obscure interception with a system rule

[java] JUnit test for System.out.println()

Examples related to java

Examples related to console

Examples related to junit