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JUnit 4’s ErrorCollector rule lets a test continue after a collected check fails, then report the collected problems together when the test finishes. Use it when checks are independent—for example, when validating several rows or fields and you want to see all their failures in one run.
Declare ErrorCollector as a JUnit 4 rule
Declare an ErrorCollector field with @Rule, then call its methods from a test. This example uses Hamcrest matchers and labels each check so a failure identifies the row it concerns:
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ErrorCollector;
import static org.hamcrest.CoreMatchers.is;
public class TableTest {
@Rule
public ErrorCollector collector = new ErrorCollector();
@Test
public void checksSeveralRows() {
String actualFirst = "ready";
String actualSecond = "pending";
collector.checkThat("first row", actualFirst, is("ready"));
collector.checkThat("second row", actualSecond, is("ready"));
}
}The first check passes and the second fails, but the test body can continue to later collected checks. At the rule’s final verification, JUnit fails the test and reports the collected problem or problems. ErrorCollector has been part of JUnit since 4.7.
The example assumes JUnit 4 and Hamcrest are available on the test classpath. Keep the checks independent: if a later operation requires an earlier assertion to have succeeded, collecting that earlier failure may leave the later operation with invalid input.
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Choose the method that matches the check
Use checkThat for matcher assertions
checkThat(value, matcher) collects a matcher failure. The overload checkThat(reason, value, matcher) adds a reason to help locate the failed condition. Prefer a specific label such as a row number or field name over a generic message.
Use addError for an existing throwable
If code has already produced a Throwable that you want included in the final report, pass it to addError:
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collector.addError(new Throwable("first thing went wrong"));
collector.addError(new Throwable("second thing went wrong"));Use this when you are collecting an error that already exists, rather than writing a matcher check.
Use checkSucceeds for code that may throw
checkSucceeds(Callable<T>) runs a callable and returns its result if it succeeds. If the callable throws, the throwable is collected and the method returns null. Do not rely on a non-null result after an exception; handle that possibility before using the returned value.
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String result = collector.checkSucceeds(() -> loadValue());
if (result != null) {
collector.checkThat("loaded value", result, is("expected"));
}In JUnit 4.13, the checkThat methods also route their assertion work through checkSucceeds. The collector therefore records thrown failures from work it wraps, not only ordinary matcher mismatches.
What ErrorCollector does—and does not—collect
The rule gathers failures passed through its collector methods and verifies the collected list after the test body. It does not make every exception in a test automatically collectable: do not assume an exception from an unrelated statement will be added to the collector. Wrap potentially throwing work with checkSucceeds, or explicitly pass an existing throwable to addError.
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ErrorCollector is a JUnit 4 rule, documented since JUnit 4.7, and remains present in JUnit 4.13. For projects moving to JUnit 6, treat it as a legacy JUnit 4 API: the available JUnit 6 guide reference identifies it in that context but does not establish setup or compatibility requirements for every project. Check your project’s actual test framework and migration requirements rather than assuming a JUnit 4 rule works unchanged under every runner.
Troubleshoot common problems
- The test stops before later checks. Confirm the failing assertion is being made through
collector.checkThator another collector method, rather than as a direct assertion outside the collector. - A thrown exception is missing from the collected report. Pass existing throwables to
addErroror run the potentially throwing operation throughcheckSucceeds. An exception from an unrelated test-body statement is not automatically collected. - A later check throws a null-related error. If a
checkSucceedscallable throws, its result isnull. Guard against that before using the return value. - The report does not identify which item failed. Use the reason-taking
checkThatoverload and give each independent check a distinct row, field, or condition label. - You are unsure how it interacts with another rule or runner. Do not assume universal behavior: the API and implementation described here do not establish every custom runner or rule interaction. Verify the behavior with the runner and configuration your project uses.
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