Class SystemOutRule
- All Implemented Interfaces:
org.junit.rules.TestRule
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 } }
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Field Summary
Fields -
Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionorg.junit.runners.model.Statement
apply
(org.junit.runners.model.Statement base, org.junit.runner.Description description) void
clearLog()
Clears the current log.Start logging of everything that is written toSystem.out
.getLog()
Returns the text that is written toSystem.out
sinceenableLog()
(respectivelyclearLog()
has been called.byte[]
Returns the raw bytes that are written toSystem.out
sinceenableLog()
(respectivelyclearLog()
has been called.Returns the text that is written toSystem.out
sinceenableLog()
(respectivelyclearLog()
has been called.mute()
Suppress the output toSystem.out
.Suppress the output toSystem.out
for successful tests only.
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Field Details
-
logPrintStream
-
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Constructor Details
-
SystemOutRule
public SystemOutRule()
-
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Method Details
-
mute
Suppress the output toSystem.out
.- Returns:
- the rule itself.
-
muteForSuccessfulTests
Suppress the output toSystem.out
for successful tests only. The output is still written toSystem.out
for failing tests.- Returns:
- the rule itself.
-
clearLog
public void clearLog()Clears the current log. -
getLog
Returns the text that is written toSystem.out
sinceenableLog()
(respectivelyclearLog()
has been called.- Returns:
- the text that is written to
System.out
sinceenableLog()
(respectivelyclearLog()
has been called.
-
getLogWithNormalizedLineSeparator
Returns the text that is written toSystem.out
sinceenableLog()
(respectivelyclearLog()
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 toSystem.out
sinceenableLog()
(respectivelyclearLog()
has been called.- Returns:
- the raw bytes that are written to
System.out
sinceenableLog()
(respectivelyclearLog()
has been called.
-
enableLog
Start logging of everything that is written toSystem.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 interfaceorg.junit.rules.TestRule
-