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Mats^3 Spring-specific testing tools, for quickly firing up a test-harness using either JUnit or Jupiter (JUnit 5).
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package io.mats3.spring.test;

import java.lang.annotation.Annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Field;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

import org.springframework.beans.factory.config.AutowireCapableBeanFactory;
import org.springframework.test.context.TestContext;
import org.springframework.test.context.TestExecutionListener;
import org.springframework.test.context.TestExecutionListeners;
import org.springframework.test.context.TestExecutionListeners.MergeMode;
import org.springframework.test.context.support.AbstractTestExecutionListener;

import io.mats3.spring.test.SpringInjectRulesAndExtensions.SpringInjectRulesAndExtensionsTestExecutionListener;

/**
 * Use this Test Execution Listener to autowire JUnit Rules and Jupiter Extensions, i.e. so that any fields in the Rule
 * or Extension annotated with @Inject or @Autowire will be autowired - typically needed for
 * Rule_MatsEndpoint and Extension_MatsEndpoint.
 * 
 * To use, just put this annotation on the test class. If that fails, typically because you are also employing a
 * different TestExecutionListener, a fallback is to directly list the
 * {@link SpringInjectRulesAndExtensionsTestExecutionListener} in the @TestExecutionListeners annotation on
 * the test class, as such:
 *
 * 
 * {@literal @}TestExecutionListeners(listeners = SpringInjectRulesAndExtensionsTestExecutionListener.class, mergeMode = MergeMode.MERGE_WITH_DEFAULTS)
 * 
 *
 * @author Kevin Mc Tiernan, 2020-11-03, [email protected]
 * @author Endre Stølsvik 2020-11-24 23:16 - http://stolsvik.com/, [email protected]
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@TestExecutionListeners(listeners = SpringInjectRulesAndExtensionsTestExecutionListener.class, mergeMode = MergeMode.MERGE_WITH_DEFAULTS)
@Documented
public @interface SpringInjectRulesAndExtensions {

    /**
     * This {@link TestExecutionListener} finds all fields in the test class instance which is annotated with JUnit
     * Rule or JUnit 5 Jupiter RegisterExtension, and runs
     * beanFactory.autowireBean(fieldValue) on them.
     */
    class SpringInjectRulesAndExtensionsTestExecutionListener extends AbstractTestExecutionListener {

        /**
         * Performs dependency injection on Rule and @RegisterExtension fields in test-class
         * as supplied by testContext.
         */
        @Override
        public void prepareTestInstance(TestContext testContext) {
            AutowireCapableBeanFactory beanFactory = testContext.getApplicationContext()
                    .getAutowireCapableBeanFactory();

            // Get all fields in test class annotated with @Rule or @RegisterExtension
            Set testRuleFields = findFields(testContext.getTestClass(), "org.junit.Rule");
            Set testExtensionFields = findFields(testContext.getTestClass(),
                    "org.junit.jupiter.api.extension.RegisterExtension");

            Set allFields = new HashSet<>();
            allFields.addAll(testRuleFields);
            allFields.addAll(testExtensionFields);

            // Use bean factory to autowire all extensions
            for (Field testField : allFields) {
                try {
                    // We need to set this accessible, even for public fields, as Java 17s access controls will
                    // prevent access otherwise.
                    testField.setAccessible(true);
                    Object ruleObject = testField.get(testContext.getTestInstance());
                    beanFactory.autowireBean(ruleObject);
                }
                catch (Exception e) {
                    throw new AssertionError("Failed read field [" + testField + "] in [" + testField.getClass() + "],"
                            + " unable to autowire it as a bean in test class [" + testContext.getTestClass() + "]", e);
                }
            }
        }

        /**
         * Find all fields in class with given annotation. Amended from StackOverflow, inspired by Apache Commons Lang.
         * 
         * https://stackoverflow.com/a/29766135
         */
        @SuppressWarnings("unchecked")
        protected static Set findFields(Class clazz, String annotationClassName) {
            try {
                Class annotationClass = (Class) Class.forName(
                        annotationClassName);
                Set set = new HashSet<>();
                Class c = clazz;
                while (c != null) {
                    for (Field field : c.getDeclaredFields()) {
                        if (field.isAnnotationPresent(annotationClass)) {
                            set.add(field);
                        }
                    }
                    c = c.getSuperclass();
                }
                return set;
            }
            catch (ClassNotFoundException e) {
                return Collections.emptySet();
            }
        }
    }
}