Use ContextCustomizerFactory to set up inprocess testing

This is the approach we use in 1.0 and it works better for users
because they can still use `@AutoConfigureInProcessTransport` even
if the test is not a `@SpringBootTest`.

Author: dsyer

samples/grpc-server/src/test/java/org/springframework/grpc/sample/GrpcServerSideTests.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2025-current the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.grpc.sample;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.context.TestConfiguration;
+import org.springframework.context.annotation.Import;
+import org.springframework.grpc.sample.GrpcServerSideTests.TestConfig;
+import org.springframework.grpc.sample.proto.HelloReply;
+import org.springframework.grpc.sample.proto.HelloRequest;
+import org.springframework.grpc.sample.proto.SimpleGrpc.SimpleBlockingStub;
+import org.springframework.grpc.test.AutoConfigureInProcessTransport;
+import org.springframework.test.context.TestPropertySource;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+/**
+ * A test that doesn't use {@link SpringBootTest @SpringBootTest}, but can still use
+ * {@link AutoConfigureInProcessTransport @AutoConfigureInProcessTransport} to set up an
+ * in-process gRPC server for testing.
+ */
+@TestPropertySource(properties = { "spring.grpc.client.default-channel.address=localhost:9090" })
+@SpringJUnitConfig(TestConfig.class)
+@AutoConfigureInProcessTransport
+public class GrpcServerSideTests {
+
+	@Autowired
+	private SimpleBlockingStub stub;
+
+	@Test
+	void contextLoads() {
+		HelloRequest request = HelloRequest.newBuilder().setName("Test").build();
+		HelloReply reply = this.stub.sayHello(request);
+		assertThat(reply.getMessage()).contains(("Test"));
+	}
+
+	@TestConfiguration
+	@Import(GrpcServerService.class)
+	@EnableAutoConfiguration
+	static class TestConfig {
+
+	}
+
+}

spring-grpc-test/src/main/java/org/springframework/grpc/test/InProcessTransportContextCustomizerFactory.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2024-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.grpc.test;
+
+import java.util.List;
+import java.util.Objects;
+
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.boot.test.util.TestPropertyValues;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.test.context.ContextConfigurationAttributes;
+import org.springframework.test.context.ContextCustomizer;
+import org.springframework.test.context.ContextCustomizerFactory;
+import org.springframework.test.context.MergedContextConfiguration;
+import org.springframework.test.context.TestContextAnnotationUtils;
+
+/**
+ * {@link ContextCustomizerFactory} that starts an in-process gRPC server and replaces the
+ * regular server and channel factories (e.g. Netty). The customizer can be disabled via
+ * the {@link AutoConfigureInProcessTransport} annotation or the
+ * {@value #ENABLED_PROPERTY} property.
+ *
+ * @author Chris Bono
+ */
+class InProcessTransportContextCustomizerFactory implements ContextCustomizerFactory {
+
+	static final String ENABLED_PROPERTY = "spring.grpc.test.inprocess.enabled";
+
+	@Override
+	public ContextCustomizer createContextCustomizer(Class<?> testClass,
+			List<ContextConfigurationAttributes> configAttributes) {
+		AutoConfigureInProcessTransport annotation = TestContextAnnotationUtils.findMergedAnnotation(testClass,
+				AutoConfigureInProcessTransport.class);
+		return new InProcessTransportContextCustomizer(annotation);
+	}
+
+	private static class InProcessTransportContextCustomizer implements ContextCustomizer {
+
+		private final @Nullable AutoConfigureInProcessTransport annotation;
+
+		InProcessTransportContextCustomizer(@Nullable AutoConfigureInProcessTransport annotation) {
+			this.annotation = annotation;
+		}
+
+		@Override
+		public void customizeContext(ConfigurableApplicationContext context,
+				MergedContextConfiguration mergedContextConfiguration) {
+			if (this.annotation == null
+					|| !context.getEnvironment().getProperty(ENABLED_PROPERTY, Boolean.class, true)) {
+				return;
+			}
+			TestPropertyValues
+				.of("spring.grpc.test.inprocess.enabled=true", "spring.grpc.client.inprocess.exclusive=true",
+						"spring.grpc.server.inprocess.exclusive=true")
+				.applyTo(context);
+		}
+
+		@Override
+		public boolean equals(@Nullable Object o) {
+			if (this == o) {
+				return true;
+			}
+			if (o == null || getClass() != o.getClass()) {
+				return false;
+			}
+			InProcessTransportContextCustomizer that = (InProcessTransportContextCustomizer) o;
+			return Objects.equals(this.annotation, that.annotation);
+		}
+
+		@Override
+		public int hashCode() {
+			return Objects.hash(this.annotation);
+		}
+
+	}
+
+}

spring-grpc-test/src/main/java/org/springframework/grpc/test/InProcessTransportEnvironmentPostProcessor.java

@@ -2,6 +2,6 @@
 org.springframework.context.ApplicationContextInitializer=\
 org.springframework.grpc.test.ServerPortInfoApplicationContextInitializer
 
-# Environment Post Processors
-org.springframework.boot.env.EnvironmentPostProcessor=\
-  org.springframework.grpc.test.InProcessTransportEnvironmentPostProcessor
+# Spring Test Context Customizer Factories
+org.springframework.test.context.ContextCustomizerFactory=\
+org.springframework.grpc.test.InProcessTransportContextCustomizerFactory