JUnit5 5.4.0 broke Android support

[matejdro]

JUnit 5.4.0 started crashing on Android due to unsupported UNICODE_CHARACTER_CLASS flag in Pattern.

Steps to reproduce

1. Create brand new emulator from Android Studio with Android 9.0 (Google Play) x86_64 image
2. checkout https://github.com/mannodermaus/android-junit5/tree/7b9f490d9ba6e7eb6b91b99e6d2755c546354e4f
3. Start emulator
4. Run ./gradlew :sample:connectedExperimentalDebugAndroidTest from the root of checked out repo
5. Tests will crash with following exception:

java.lang.ExceptionInInitializerError
at org.junit.platform.commons.util.StringUtils.isNotBlank(StringUtils.java:64)
at org.junit.platform.commons.util.Preconditions.notBlank(Preconditions.java:248)
at org.junit.platform.launcher.core.LauncherConfigurationParameters.<init>(LauncherConfigurationParameters.java:44)
at org.junit.platform.launcher.core.LauncherConfigurationParameters.<init>(LauncherConfigurationParameters.java:39)
at org.junit.platform.launcher.core.LauncherDiscoveryRequestBuilder.build(LauncherDiscoveryRequestBuilder.java:194)
at org.junit.platform.runner.JUnitPlatform.createDiscoveryRequest(JUnitPlatform.java:153)
at org.junit.platform.runner.JUnitPlatform.<init>(JUnitPlatform.java:124)
at org.junit.platform.runner.JUnitPlatform.<init>(JUnitPlatform.java:117)
at de.mannodermaus.junit5.AndroidJUnit5.<init>(Runner.kt:17)
at de.mannodermaus.junit5.RunnerKt.createJUnit5Runner(Runner.kt:22)
at de.mannodermaus.junit5.AndroidJUnit5Builder.runnerForClass(RunnerBuilder.kt:69)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at androidx.test.internal.runner.AndroidRunnerBuilder.runnerForClass(AndroidRunnerBuilder.java:147)
at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
at androidx.test.internal.runner.TestLoader.doCreateRunner(TestLoader.java:73)
at androidx.test.internal.runner.TestLoader.getRunnersFor(TestLoader.java:104)
at androidx.test.internal.runner.TestRequestBuilder.build(TestRequestBuilder.java:789)
at androidx.test.runner.AndroidJUnitRunner.buildRequest(AndroidJUnitRunner.java:543)
at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:386)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:2145)
Caused by: java.lang.IllegalArgumentException: Unsupported flags: 256
at java.util.regex.Pattern.<init>(Pattern.java:1324)
at java.util.regex.Pattern.compile(Pattern.java:975)
at org.junit.platform.commons.util.StringUtils.<clinit>(StringUtils.java:36)
... 20 more

It looks like UNICODE_CHARACTER_CLASS was added to StringUtils in JUnit 5.4.0. This flag is not supported on Android (not even on the latest devices) and causes crash whenever StringUtils class is initialized.

[sormuras]

I remember that late change in line db8b3d4#diff-6620c2fff8288084514a095acc4cb5a8R36 ... used another in-line regular expression before.

[sormuras]

The constant is available since Java 1.7, though. Copied from java.util.regex.Pattern#UNICODE_CHARACTER_CLASS:

    /**
     * Enables the Unicode version of <i>Predefined character classes</i> and
     * <i>POSIX character classes</i>.
     *
     * <p> When this flag is specified then the (US-ASCII only)
     * <i>Predefined character classes</i> and <i>POSIX character classes</i>
     * are in conformance with
     * <a href="http://www.unicode.org/reports/tr18/"><i>Unicode Technical
     * Standard #18: Unicode Regular Expression</i></a>
     * <i>Annex C: Compatibility Properties</i>.
     * <p>
     * The UNICODE_CHARACTER_CLASS mode can also be enabled via the embedded
     * flag expression&nbsp;{@code (?U)}.
     * <p>
     * The flag implies UNICODE_CASE, that is, it enables Unicode-aware case
     * folding.
     * <p>
     * Specifying this flag may impose a performance penalty.  </p>
     * @since 1.7
     */
    public static final int UNICODE_CHARACTER_CLASS = 0x100;

[sormuras]

Reading the documention again...

     * The UNICODE_CHARACTER_CLASS mode can also be enabled via the embedded
     * flag expression {@code (?U)}.

...(?U) is the pattern snippet, aka flag expression.

[sormuras]

@matejdro Does Android support (?U)?

[matejdro]

The constant is available since Java 1.7, though

Yes, the constant itself is available. Unfortunately it appears Android folks forgot to update validation code, so this constant is still not allowed to be passed into Pattern constructor.

Does Android support (?U)?

Just checked and Android 9.0 (latest one at the moment) throws syntax error when I try to create Pattern with this text added in. On the bright side, this mode appears to be always enabled on Android according to Android Documentation.

[sormuras]

Thanks for checking @matejdro -- mh, maybe we have to resort to only look for non-Unicode control chars on Android. Perhaps we'll enhance the OS enumeration (is it possible to determine Android OS at runtime?) with an ANDROID constant.

Marked for @junit-team discussion.

[sormuras]

...or guard the compilation of the patterns in StringUtils https://github.com/junit-team/junit5/blob/master/junit-platform-commons/src/main/java/org/junit/platform/commons/util/StringUtils.java#L36-L37 in a static initializer and fall-back to a non-Unicode one.