Make ApplicationContextRunner immutable

Update `ApplicationContextRunner` so that it is totally immutable.
Methods now return new instances rather than changing existing state.

See gh-9875

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jms/artemis/ArtemisAutoConfigurationTests.java

@@ -231,15 +231,15 @@ public class ArtemisAutoConfigurationTests {
 						"spring.artemis.embedded.dataDirectory:"
 								+ dataFolder.getAbsolutePath())
 				.run((context) -> context.getBean(JmsTemplate.class).send("TestQueue",
-						(session) -> session.createTextMessage(messageId)));
-		// Start the server again and check if our message is still here
-		this.contextRunner.run((context) -> {
-			JmsTemplate jmsTemplate2 = context.getBean(JmsTemplate.class);
-			jmsTemplate2.setReceiveTimeout(1000L);
-			Message message = jmsTemplate2.receive("TestQueue");
-			assertThat(message).isNotNull();
-			assertThat(((TextMessage) message).getText()).isEqualTo(messageId);
-		});
+						(session) -> session.createTextMessage(messageId)))
+				.run((context) -> {
+					// Start the server again and check if our message is still here
+					JmsTemplate jmsTemplate2 = context.getBean(JmsTemplate.class);
+					jmsTemplate2.setReceiveTimeout(1000L);
+					Message message = jmsTemplate2.receive("TestQueue");
+					assertThat(message).isNotNull();
+					assertThat(((TextMessage) message).getText()).isEqualTo(messageId);
+				});
 	}
 
 	@Test

spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/jdbc/TestDatabaseAutoConfigurationTests.java

@@ -55,7 +55,9 @@ public class TestDatabaseAutoConfigurationTests {
 					DataSource datasource = context.getBean(DataSource.class);
 					JdbcTemplate jdbcTemplate = new JdbcTemplate(datasource);
 					jdbcTemplate.execute("create table example (id int, name varchar);");
-					this.contextRunner.run((secondContext) -> {
+					this.contextRunner
+							.withUserConfiguration(ExistingDataSourceConfiguration.class)
+							.run((secondContext) -> {
 						DataSource anotherDatasource = secondContext
 								.getBean(DataSource.class);
 						JdbcTemplate anotherJdbcTemplate = new JdbcTemplate(

spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.test.context.runner;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Supplier;
 
@@ -98,25 +99,48 @@ abstract class AbstractApplicationContextRunner<SELF extends AbstractApplication
 
 	private final Supplier<C> contextFactory;
 
-	private TestPropertyValues environmentProperties;
+	private final TestPropertyValues environmentProperties;
 
-	private TestPropertyValues systemProperties;
+	private final TestPropertyValues systemProperties;
 
-	private ClassLoader classLoader;
+	private final ClassLoader classLoader;
 
-	private ApplicationContext parent;
+	private final ApplicationContext parent;
 
-	private final List<Configurations> configurations = new ArrayList<>();
+	private final List<Configurations> configurations;
 
 	/**
 	 * Create a new {@link AbstractApplicationContextRunner} instance.
 	 * @param contextFactory the factory used to create the actual context
 	 */
 	protected AbstractApplicationContextRunner(Supplier<C> contextFactory) {
+		this(contextFactory, TestPropertyValues.empty(), TestPropertyValues.empty(), null,
+				null, Collections.emptyList());
+	}
+
+	/**
+	 * Create a new {@link AbstractApplicationContextRunner} instance.
+	 * @param contextFactory the factory used to create the actual context
+	 * @param environmentProperties the environment properties
+	 * @param systemProperties the system properties
+	 * @param classLoader the class loader
+	 * @param parent the parent
+	 * @param configurations the configuration
+	 */
+	protected AbstractApplicationContextRunner(Supplier<C> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
 		Assert.notNull(contextFactory, "ContextFactory must not be null");
+		Assert.notNull(environmentProperties, "EnvironmentProperties must not be null");
+		Assert.notNull(systemProperties, "SystemProperties must not be null");
+		Assert.notNull(configurations, "Configurations must not be null");
 		this.contextFactory = contextFactory;
-		this.environmentProperties = TestPropertyValues.empty();
-		this.systemProperties = TestPropertyValues.empty();
+		this.environmentProperties = environmentProperties;
+		this.systemProperties = systemProperties;
+		this.classLoader = classLoader;
+		this.parent = parent;
+		this.configurations = Collections.unmodifiableList(configurations);
 	}
 
 	/**
@@ -125,13 +149,14 @@ abstract class AbstractApplicationContextRunner<SELF extends AbstractApplication
 	 * might have been specified previously.
 	 * @param pairs the key-value pairs for properties that need to be added to the
 	 * environment
-	 * @return this instance
+	 * @return a new instance with the updated property values
 	 * @see TestPropertyValues
 	 * @see #withSystemProperties(String...)
 	 */
 	public SELF withPropertyValues(String... pairs) {
-		this.environmentProperties = this.environmentProperties.and(pairs);
-		return self();
+		return newInstance(this.contextFactory, this.environmentProperties.and(pairs),
+				this.systemProperties, this.classLoader, this.parent,
+				this.configurations);
 	}
 
 	/**
@@ -140,13 +165,14 @@ abstract class AbstractApplicationContextRunner<SELF extends AbstractApplication
 	 * context is {@link #run(ContextConsumer) run} and restored when the context is
 	 * closed.
 	 * @param pairs the key-value pairs for properties that need to be added to the system
-	 * @return this instance
+	 * @return a new instance with the updated system properties
 	 * @see TestPropertyValues
 	 * @see #withSystemProperties(String...)
 	 */
 	public SELF withSystemProperties(String... pairs) {
-		this.systemProperties = this.systemProperties.and(pairs);
-		return self();
+		return newInstance(this.contextFactory, this.environmentProperties,
+				this.systemProperties.and(pairs), this.classLoader, this.parent,
+				this.configurations);
 	}
 
 	/**
@@ -154,30 +180,30 @@ abstract class AbstractApplicationContextRunner<SELF extends AbstractApplication
 	 * Customizing the {@link ClassLoader} is an effective manner to hide resources from
 	 * the classpath.
 	 * @param classLoader the classloader to use (can be null to use the default)
-	 * @return this instance
+	 * @return a new instance with the updated class loader
 	 * @see HidePackagesClassLoader
 	 */
 	public SELF withClassLoader(ClassLoader classLoader) {
-		this.classLoader = classLoader;
-		return self();
+		return newInstance(this.contextFactory, this.environmentProperties,
+				this.systemProperties, classLoader, this.parent, this.configurations);
 	}
 
 	/**
 	 * Configure the {@link ConfigurableApplicationContext#setParent(ApplicationContext)
 	 * parent} of the {@link ApplicationContext}.
 	 * @param parent the parent
-	 * @return this instance
+	 * @return a new instance with the updated parent
 	 */
 	public SELF withParent(ApplicationContext parent) {
-		this.parent = parent;
-		return self();
+		return newInstance(this.contextFactory, this.environmentProperties,
+				this.systemProperties, this.classLoader, parent, this.configurations);
 	}
 
 	/**
 	 * Register the specified user configuration classes with the
 	 * {@link ApplicationContext}.
 	 * @param configurationClasses the user configuration classes to add
-	 * @return this instance
+	 * @return a new instance with the updated configuration
 	 */
 	public SELF withUserConfiguration(Class<?>... configurationClasses) {
 		return withConfiguration(UserConfigurations.of(configurationClasses));
@@ -186,32 +212,42 @@ abstract class AbstractApplicationContextRunner<SELF extends AbstractApplication
 	/**
 	 * Register the specified configuration classes with the {@link ApplicationContext}.
 	 * @param configurations the configurations to add
-	 * @return this instance
+	 * @return a new instance with the updated configuration
 	 */
 	public SELF withConfiguration(Configurations configurations) {
 		Assert.notNull(configurations, "Configurations must not be null");
-		this.configurations.add(configurations);
-		return self();
+		return newInstance(this.contextFactory, this.environmentProperties,
+				this.systemProperties, this.classLoader, this.parent,
+				add(this.configurations, configurations));
 	}
 
-	@SuppressWarnings("unchecked")
-	protected final SELF self() {
-		return (SELF) this;
+	private <T> List<T> add(List<T> list, T element) {
+		List<T> result = new ArrayList<>(list);
+		result.add(element);
+		return result;
 	}
 
+	protected abstract SELF newInstance(Supplier<C> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations);
+
 	/**
 	 * Create and refresh a new {@link ApplicationContext} based on the current state of
 	 * this loader. The context is consumed by the specified {@code consumer} and closed
 	 * upon completion.
 	 * @param consumer the consumer of the created {@link ApplicationContext}
+	 * @return this instance
 	 */
-	public void run(ContextConsumer<? super A> consumer) {
+	@SuppressWarnings("unchecked")
+	public SELF run(ContextConsumer<? super A> consumer) {
 		this.systemProperties.applyToSystemProperties(() -> {
 			try (A context = createAssertableContext()) {
 				accept(consumer, context);
 			}
 			return null;
 		});
+		return (SELF) this;
 	}
 
 	@SuppressWarnings("unchecked")

spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ApplicationContextRunner.java

@@ -16,9 +16,13 @@
 
 package org.springframework.boot.test.context.runner;
 
+import java.util.List;
 import java.util.function.Supplier;
 
+import org.springframework.boot.context.annotation.Configurations;
 import org.springframework.boot.test.context.assertj.AssertableApplicationContext;
+import org.springframework.boot.test.util.TestPropertyValues;
+import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 
@@ -54,4 +58,23 @@ public class ApplicationContextRunner extends
 		super(contextFactory);
 	}
 
+	private ApplicationContextRunner(
+			Supplier<ConfigurableApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		super(contextFactory, environmentProperties, systemProperties, classLoader,
+				parent, configurations);
+	}
+
+	@Override
+	protected ApplicationContextRunner newInstance(
+			Supplier<ConfigurableApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		return new ApplicationContextRunner(contextFactory, environmentProperties,
+				systemProperties, classLoader, parent, configurations);
+	}
+
 }

spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/ReactiveWebApplicationContextRunner.java

@@ -16,11 +16,15 @@
 
 package org.springframework.boot.test.context.runner;
 
+import java.util.List;
 import java.util.function.Supplier;
 
+import org.springframework.boot.context.annotation.Configurations;
 import org.springframework.boot.test.context.assertj.AssertableReactiveWebApplicationContext;
+import org.springframework.boot.test.util.TestPropertyValues;
 import org.springframework.boot.web.reactive.context.ConfigurableReactiveWebApplicationContext;
 import org.springframework.boot.web.reactive.context.GenericReactiveWebApplicationContext;
+import org.springframework.context.ApplicationContext;
 
 /**
  * A {@link AbstractApplicationContextRunner ApplicationContext runner} for a
@@ -54,4 +58,24 @@ public final class ReactiveWebApplicationContextRunner extends
 		super(contextFactory);
 	}
 
+	private ReactiveWebApplicationContextRunner(
+			Supplier<ConfigurableReactiveWebApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		super(contextFactory, environmentProperties, systemProperties, classLoader,
+				parent, configurations);
+	}
+
+	@Override
+	protected ReactiveWebApplicationContextRunner newInstance(
+			Supplier<ConfigurableReactiveWebApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		return new ReactiveWebApplicationContextRunner(contextFactory,
+				environmentProperties, systemProperties, classLoader, parent,
+				configurations);
+	}
+
 }

spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java

@@ -16,9 +16,13 @@
 
 package org.springframework.boot.test.context.runner;
 
+import java.util.List;
 import java.util.function.Supplier;
 
+import org.springframework.boot.context.annotation.Configurations;
 import org.springframework.boot.test.context.assertj.AssertableWebApplicationContext;
+import org.springframework.boot.test.util.TestPropertyValues;
+import org.springframework.context.ApplicationContext;
 import org.springframework.mock.web.MockServletContext;
 import org.springframework.web.context.ConfigurableWebApplicationContext;
 import org.springframework.web.context.WebApplicationContext;
@@ -58,6 +62,25 @@ public final class WebApplicationContextRunner extends
 		super(contextFactory);
 	}
 
+	private WebApplicationContextRunner(
+			Supplier<ConfigurableWebApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		super(contextFactory, environmentProperties, systemProperties, classLoader,
+				parent, configurations);
+	}
+
+	@Override
+	protected WebApplicationContextRunner newInstance(
+			Supplier<ConfigurableWebApplicationContext> contextFactory,
+			TestPropertyValues environmentProperties, TestPropertyValues systemProperties,
+			ClassLoader classLoader, ApplicationContext parent,
+			List<Configurations> configurations) {
+		return new WebApplicationContextRunner(contextFactory, environmentProperties,
+				systemProperties, classLoader, parent, configurations);
+	}
+
 	/**
 	 * Decorate the specified {@code contextFactory} to set a {@link MockServletContext}
 	 * on each newly created {@link WebApplicationContext}.