Package org.junit.contrib.java.lang.system

Class SystemOutRule

java.lang.Object

org.junit.contrib.java.lang.system.SystemOutRule

All Implemented Interfaces:

org.junit.rules.TestRule

public class SystemOutRule
extends java.lang.Object
implements org.junit.rules.TestRule

The SystemOutRule intercepts the writes to System.out. It is used to make assertions about the text that is written to System.out or to mute System.out.

Assertions

SystemOutRule may be used for verifying the text that is written to System.out.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

   @Test
   public void test() {
     System.out.print("some text");
     assertEquals("some text", systemOutRule.getLog());
   }
 }
 

If your code under test writes the correct new line characters to System.out then the test output is different at different systems. getLogWithNormalizedLineSeparator() provides a log that always uses \n as line separator. This makes it easy to write appropriate assertions that work on all systems.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

   @Test
   public void test() {
     System.out.print(String.format("some text%n"));
     assertEquals("some text\n", systemOutRule.getLogWithNormalizedLineSeparator());
   }
 }
 

If your code under test writes raw binary data to System.out then you can read it by means of getLogAsBytes()).

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

   @Test
   public void test() {
     byte[] data = { 1, 2, 3, 4, 5 };
     System.out.write(data, 0, data.length);
     assertEquals(data, systemOutRule.getLogAsBytes());
   }
 }
 

You don't have to enable logging for every test. It can be enabled for specific tests only.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule();

   @Test
   public void testWithLogging() {
     systemOutRule.enableLog()
     System.out.print("some text");
     assertEquals("some text", systemOutRule.getLog());
   }

   @Test
   public void testWithoutLogging() {
     System.out.print("some text");
   }
 }
 

If you want to verify parts of the output only then you can clear the log during a test.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().enableLog();

   @Test
   public void test() {
     System.out.print("uninteresting things");
     systemOutRule.clearLog()
     System.out.print("interesting things");
     assertEquals("interesting things", systemOutRule.getLog());
   }
 }
 

Muting

Usually the output of a test to System.out does not have to be visible. It may even slowdown the test. SystemOutRule can be used to suppress this output.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().mute();

   @Test
   public void test() {
     System.out.print("some text"); //is not visible on the console
   }
 }
 

You don't have to mute System.out for every test. It can be muted for specific tests only.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule();

   @Test
   public void testWithSuppressedOutput() {
     systemOutRule.mute()
     System.out.print("some text");
   }

   @Test
   public void testWithNormalOutput() {
     System.out.print("some text");
   }
 }
 

In case of a failed test it is sometimes helpful to see the output. This is when the method muteForSuccessfulTests() comes into play.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().muteForSuccessfulTests();

   @Test
   public void testWithSuppressedOutput() {
     System.out.print("some text");
   }

   @Test
   public void testWithNormalOutput() {
     System.out.print("some text");
     fail();
   }
 }
 

Combine Logging and Muting

Logging and muting can be combined. No output is actually written to System.out but everything that would have been written is available from the log.

 public class SystemOutTest {
   @Rule
   public final SystemOutRule systemOutRule = new SystemOutRule().mute().enableLog();

   @Test
   public void test() {
     System.out.print("some text"); //is not visible on the console
     assertEquals("some text", systemOutRule.getLog()); //succeeds
   }
 }
 

Constructor Summary

Constructors

Constructor	Description
SystemOutRule()

Method Summary

Modifier and Type	Method	Description
org.junit.runners.model.Statement	apply(org.junit.runners.model.Statement base, org.junit.runner.Description description)
void	clearLog()	Clears the current log.
SystemOutRule	enableLog()	Start logging of everything that is written to System.out.
java.lang.String	getLog()	Returns the text that is written to System.out since enableLog() (respectively clearLog() has been called.
byte[]	getLogAsBytes()	Returns the raw bytes that are written to System.out since enableLog() (respectively clearLog() has been called.
java.lang.String	getLogWithNormalizedLineSeparator()	Returns the text that is written to System.out since enableLog() (respectively clearLog() has been called.
SystemOutRule	mute()	Suppress the output to System.out.
SystemOutRule	muteForSuccessfulTests()	Suppress the output to System.out for successful tests only.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

SystemOutRule

public SystemOutRule()

Method Detail

mute

public SystemOutRule mute()

Suppress the output to System.out.

Returns:

the rule itself.

muteForSuccessfulTests

public SystemOutRule muteForSuccessfulTests()

Suppress the output to System.out for successful tests only. The output is still written to System.out for failing tests.

Returns:

the rule itself.

clearLog

public void clearLog()

Clears the current log.

getLog

public java.lang.String getLog()

Returns the text that is written to System.out since enableLog() (respectively clearLog() has been called.

Returns:

the text that is written to System.out since enableLog() (respectively clearLog() has been called.

getLogWithNormalizedLineSeparator

public java.lang.String getLogWithNormalizedLineSeparator()

Returns the text that is written to System.out since enableLog() (respectively clearLog() has been called. New line characters are replaced with a single \n.

Returns:

the normalized log.

getLogAsBytes

public byte[] getLogAsBytes()

Returns the raw bytes that are written to System.out since enableLog() (respectively clearLog() has been called.

Returns:

the raw bytes that are written to System.out since enableLog() (respectively clearLog() has been called.

enableLog

public SystemOutRule enableLog()

Start logging of everything that is written to System.out.

Returns:

the rule itself.

apply

public org.junit.runners.model.Statement apply(org.junit.runners.model.Statement base,
                                               org.junit.runner.Description description)

Specified by:

apply in interface org.junit.rules.TestRule