Document best practices for implementing ParameterResolvers

documentation/src/docs/asciidoc/user-guide/extensions.adoc

@@ -484,6 +484,57 @@ constructor invocations, using the `{ExecutableInvoker}` available via the
 `getExecutableInvoker()` method in the `ExtensionContext`.
 ====
 
+[[extensions-parameter-resolution-conflicts]]
+==== Parameter Conflicts
+
+If multiple implementations of `ParameterResolver` that support the same type are
+registered for a test, a `ParameterResolutionException` will be thrown, with a
+message to indicate that competing resolvers have been discovered. See the following
+example:
+
+[source,java,indent=0]
+.Conflicting parameter resolution due to multiple resolvers claiming support for integers
+----
+include::{testDir}/example/extensions/ParameterResolverConflictDemo.java[tags=user_guide]
+----
+
+If the conflicting `ParameterResolver` implementations are applied to different test
+methods as shown in the following example, no conflict occurs.
+
+[source,java,indent=0]
+.Fine-grained registration to avoid conflict
+----
+include::{testDir}/example/extensions/ParameterResolverNoConflictDemo.java[tags=user_guide]
+----
+
+If the conflicting `ParameterResolver` implementations need to be applied to the same test
+method, you can implement a custom type or custom annotation as illustrated by
+`{CustomTypeParameterResolver}` and `{CustomAnnotationParameterResolver}`, respectively.
+
+[source,java,indent=0]
+.Custom type to resolve duplicate types
+----
+include::{testDir}/example/extensions/ParameterResolverCustomTypeDemo.java[tags=user_guide]
+----
+
+A custom annotation makes the duplicate type distinguishable from its counterpart:
+
+[source,java,indent=0]
+.Custom annotation to resolve duplicate types
+----
+include::{testDir}/example/extensions/ParameterResolverCustomAnnotationDemo.java[tags=user_guide]
+----
+
+JUnit includes some built-in parameter resolvers that can cause conflicts if a resolver
+attempts to claim their supported types. For example, `{TestInfo}` provides metadata about
+tests. See <<writing-tests-dependency-injection>> for details. Third-party frameworks such
+as Spring may also define parameter resolvers. Apply one of the techniques in this section
+to resolve any conflicts.
+
+Parameterized tests are another potential source of conflict. Ensure that tests annotated
+with `@ParameterizedTest` are not also annotated with `@Test` and see
+<<writing-tests-parameterized-tests-consuming-arguments>> for more details.
+
 [[extensions-test-result-processing]]
 === Test Result Processing
 

documentation/src/test/java/example/extensions/ParameterResolverConflictDemo.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2015-2024 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.extensions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import extensions.ExpectToFail;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.api.extension.ParameterResolver;
+
+// tag::user_guide[]
+public class ParameterResolverConflictDemo {
+
+	// end::user_guide[]
+	@ExpectToFail
+	// tag::user_guide[]
+	@Test
+	@ExtendWith({ FirstIntegerResolver.class, SecondIntegerResolver.class })
+	void testInt(int i) {
+		// Test will not run due to ParameterResolutionException
+		assertEquals(1, i);
+	}
+
+	static class FirstIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType() == int.class;
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return 1;
+		}
+	}
+
+	static class SecondIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType() == int.class;
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return 2;
+		}
+	}
+}
+// end::user_guide[]

documentation/src/test/java/example/extensions/ParameterResolverCustomAnnotationDemo.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2015-2024 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.extensions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.api.extension.ParameterResolver;
+
+// tag::user_guide[]
+public class ParameterResolverCustomAnnotationDemo {
+
+	@Test
+	void testInt(@FirstInteger Integer first, @SecondInteger Integer second) {
+		assertEquals(1, first);
+		assertEquals(2, second);
+	}
+
+	@Target(ElementType.PARAMETER)
+	@Retention(RetentionPolicy.RUNTIME)
+	@ExtendWith(FirstInteger.Extension.class)
+	public @interface FirstInteger {
+
+		class Extension implements ParameterResolver {
+
+			@Override
+			public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+				return parameterContext.getParameter().getType().equals(Integer.class)
+						&& !parameterContext.isAnnotated(SecondInteger.class);
+			}
+
+			@Override
+			public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+				return 1;
+			}
+		}
+	}
+
+	@Target(ElementType.PARAMETER)
+	@Retention(RetentionPolicy.RUNTIME)
+	@ExtendWith(SecondInteger.Extension.class)
+	public @interface SecondInteger {
+
+		class Extension implements ParameterResolver {
+
+			@Override
+			public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+				return parameterContext.isAnnotated(SecondInteger.class);
+			}
+
+			@Override
+			public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+				return 2;
+			}
+		}
+	}
+}
+// end::user_guide[]

documentation/src/test/java/example/extensions/ParameterResolverCustomTypeDemo.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2015-2024 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.extensions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.api.extension.ParameterResolver;
+
+// tag::user_guide[]
+public class ParameterResolverCustomTypeDemo {
+
+	@Test
+	@ExtendWith({ FirstIntegerResolver.class, SecondIntegerResolver.class })
+	void testInt(Integer i, WrappedInteger wrappedInteger) {
+		assertEquals(1, i);
+		assertEquals(2, wrappedInteger.value);
+	}
+
+	static class FirstIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType().equals(Integer.class);
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return 1;
+		}
+	}
+
+	static class SecondIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType().equals(WrappedInteger.class);
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return new WrappedInteger(2);
+		}
+	}
+
+	static class WrappedInteger {
+
+		private final int value;
+
+		public WrappedInteger(int value) {
+			this.value = value;
+		}
+
+	}
+}
+// end::user_guide[]

documentation/src/test/java/example/extensions/ParameterResolverNoConflictDemo.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2015-2024 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package example.extensions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ParameterContext;
+import org.junit.jupiter.api.extension.ParameterResolver;
+
+// tag::user_guide[]
+public class ParameterResolverNoConflictDemo {
+
+	@Test
+	@ExtendWith(FirstIntegerResolver.class)
+	void firstResolution(int i) {
+		assertEquals(1, i);
+	}
+
+	@Test
+	@ExtendWith(SecondIntegerResolver.class)
+	void secondResolution(int i) {
+		assertEquals(2, i);
+	}
+
+	static class FirstIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType() == int.class;
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return 1;
+		}
+	}
+
+	static class SecondIntegerResolver implements ParameterResolver {
+
+		@Override
+		public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return parameterContext.getParameter().getType() == int.class;
+		}
+
+		@Override
+		public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
+			return 2;
+		}
+	}
+}
+// end::user_guide[]