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Merged
merged 1 commit into from
Oct 28, 2025
+45 −36
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Stop swallowing System.err in JUnit tests #2685

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3 changes: 1 addition & 2 deletions tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/AllNonBrowserTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -111,8 +111,7 @@ public static void afterClass() {
}

for (Error leak : leakedResources) {
// For some reason, printing to System.err in JUnit test has no effect
leak.printStackTrace (System.out);
leak.printStackTrace ();
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@jonahgraham jonahgraham Oct 28, 2025

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FYI @SyntevoAlex I found out where System.err was going.

}

if (0 != leakedResources.size ()) {
Expand Down
24 changes: 24 additions & 0 deletions tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/SwtTestUtil.java
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Expand Up @@ -19,9 +19,11 @@
import static org.junit.jupiter.api.Assertions.assertTrue;
import static org.junit.jupiter.api.Assertions.fail;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintStream;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.HashMap;
Expand Down Expand Up @@ -764,4 +766,26 @@ static <T> T runOperationInThread(int timeoutMs, ExceptionalSupplier<T> supplier
T result = (T) supplierValue[0];
return result;
}

/**
* Capture any output on System.err
*
* This method does not capture output on stderr from C level, such as
* Gdk-CRITICAL messages.
*
* @param runnable to run while capturing output
* @return output on System.err
*/
public static String runWithCapturedStderr(Runnable runnable) {
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@mickaelistria mickaelistria Oct 31, 2025

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FWIW, I've recently relied on AutoCloseable a lot for testing; it allows some good way of wrapping state relying less on Runnable.

For this case, it could go that way

class CaptureStderr implements AutoCloseable, Supplier<String> {
  ByteArrayOutputStream errContent
  CaptureStderr() {
     errContent = new ByteArrayOutputStream();
	 System.setErr(new PrintStream(errContent, true, StandardCharsets.UTF_8));
  }
  @Override
  public void close() {
    System.setErr(stderr);
    errContent.close();
  }
  @Override
  public String get() {
    return errContent.toString(StandardCharsets.UTF_8);
  }
}

and then tests can be written

try (new captureStdErr = new CaptureStdErr()) {
   // do stuff
  assertTrue(captureStdErr.get().contains("blah");
}

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@jonahgraham jonahgraham Nov 28, 2025

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I have been meaning to come back to thank you for this advice - while I have not updated this now closed PR, I have used this methodology in my latest testing PR: #2825

PrintStream originalErr = System.err;
ByteArrayOutputStream errContent = new ByteArrayOutputStream();
System.setErr(new PrintStream(errContent, true, StandardCharsets.UTF_8));
try {
runnable.run();
return errContent.toString(StandardCharsets.UTF_8);

} finally {
System.setErr(originalErr);
}
}
}
27 changes: 10 additions & 17 deletions ...ipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_SWTError.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,6 @@
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

import org.eclipse.swt.SWT;
import org.eclipse.swt.SWTError;
import org.junit.jupiter.api.Test;
Expand Down Expand Up @@ -63,21 +60,17 @@ public void test_getMessage() {
public void test_printStackTrace() {

// test default SWTError

ByteArrayOutputStream out = new ByteArrayOutputStream();
System.setErr(new PrintStream(out));
SWTError error = new SWTError();
error.printStackTrace();
assertTrue(out.size() > 0);
assertTrue(new String(out.toByteArray()).contains("test_printStackTrace"));
SWTError error1 = new SWTError();
String stderr = SwtTestUtil.runWithCapturedStderr(() -> {
error1.printStackTrace();
});
assertTrue(stderr.contains("test_printStackTrace"));

// test SWTError with code

out = new ByteArrayOutputStream();
System.setErr(new PrintStream(out));
error = new SWTError(SWT.ERROR_INVALID_ARGUMENT);
error.printStackTrace();
assertTrue(out.size() > 0);
assertTrue(new String(out.toByteArray()).contains("test_printStackTrace"));
SWTError error2 = new SWTError(SWT.ERROR_INVALID_ARGUMENT);
stderr = SwtTestUtil.runWithCapturedStderr(() -> {
error2.printStackTrace();
});
assertTrue(stderr.contains("test_printStackTrace"));
}
}
27 changes: 10 additions & 17 deletions ....swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_SWTException.java
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Original file line number Diff line number Diff line change
Expand Up @@ -17,9 +17,6 @@
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertTrue;

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

import org.eclipse.swt.SWT;
import org.eclipse.swt.SWTException;
import org.junit.jupiter.api.Test;
Expand Down Expand Up @@ -64,21 +61,17 @@ public void test_getMessage() {
public void test_printStackTrace() {

// test default SWTException

ByteArrayOutputStream out = new ByteArrayOutputStream();
System.setErr(new PrintStream(out));
SWTException error = new SWTException();
error.printStackTrace();
assertTrue(out.size() > 0);
assertTrue(new String(out.toByteArray()).contains("test_printStackTrace"));
SWTException error1 = new SWTException();
String stderr = SwtTestUtil.runWithCapturedStderr(() -> {
error1.printStackTrace();
});
assertTrue(stderr.contains("test_printStackTrace"));

// test SWTException with code

out = new ByteArrayOutputStream();
System.setErr(new PrintStream(out));
error = new SWTException(SWT.ERROR_INVALID_ARGUMENT);
error.printStackTrace();
assertTrue(out.size() > 0);
assertTrue(new String(out.toByteArray()).contains("test_printStackTrace"));
SWTException error2 = new SWTException(SWT.ERROR_INVALID_ARGUMENT);
stderr = SwtTestUtil.runWithCapturedStderr(() -> {
error2.printStackTrace();
});
assertTrue(stderr.contains("test_printStackTrace"));
}
}
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