Fix IntegrationFlowContext concurrency issue

When we register `IntegrationFlow` s concurrently at runtime, we may
end up with the problem when we register the same object with the same
bean name, but in different places.
Or when we turn off bean overriding, we end up with the exception that
bean with the name already registered

* Wrap `IntegrationFlow` bean registration in the
`StandardIntegrationFlowContext` into the `Lock` when its bean name
is generating
* Make `StandardIntegrationFlowContext.registry` as `ConcurrentHashMap`
to avoid `ConcurrentModificationException` during `put()` and `remove()`
* Fix concurrency for beans registration with the generation names in
the `IntegrationFlowBeanPostProcessor` using an `IntegrationFlow` id
as a prefix for uniqueness.

**Cherry-pick to 5.0.x**

Fix generated bean name in the WebFluxDslTests

Use only single `Lock` in the `StandardIntegrationFlowContext`:
we don't need a fully blown `LockRegistry` there anymore since we have
only one synchronization block there and it is always around the same
type
* Add `What's New` note, and mention changes in the `dsl.adoc`

Minor doc polishing.

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/dsl/IntegrationFlowBeanPostProcessor.java

@@ -123,21 +123,18 @@ private Object processStandardIntegrationFlow(StandardIntegrationFlow flow, Stri
 				String id = endpointSpec.getId();
 
 				if (id == null) {
-					id = generateBeanName(endpoint, entry.getValue());
+					id = generateBeanName(endpoint, flowNamePrefix, entry.getValue());
 				}
 
 				Collection<?> messageHandlers =
 						this.beanFactory.getBeansOfType(messageHandler.getClass(), false, false)
 								.values();
 
 				if (!messageHandlers.contains(messageHandler)) {
-					String handlerBeanName = generateBeanName(messageHandler);
-					String[] handlerAlias = new String[] { id + IntegrationConfigUtils.HANDLER_ALIAS_SUFFIX };
+					String handlerBeanName = generateBeanName(messageHandler, flowNamePrefix);
 
 					registerComponent(messageHandler, handlerBeanName, flowBeanName);
-					for (String alias : handlerAlias) {
-						this.beanFactory.registerAlias(handlerBeanName, alias);
-					}
+					this.beanFactory.registerAlias(handlerBeanName, id + IntegrationConfigUtils.HANDLER_ALIAS_SUFFIX);
 				}
 
 				registerComponent(endpoint, id, flowBeanName);
@@ -187,12 +184,13 @@ else if (component instanceof SourcePollingChannelAdapterSpec) {
 													.values()
 													.contains(o.getKey()))
 									.forEach(o ->
-											registerComponent(o.getKey(), generateBeanName(o.getKey(), o.getValue())));
+											registerComponent(o.getKey(),
+													generateBeanName(o.getKey(), flowNamePrefix, o.getValue())));
 						}
 						SourcePollingChannelAdapterFactoryBean pollingChannelAdapterFactoryBean = spec.get().getT1();
 						String id = spec.getId();
 						if (!StringUtils.hasText(id)) {
-							id = generateBeanName(pollingChannelAdapterFactoryBean, entry.getValue());
+							id = generateBeanName(pollingChannelAdapterFactoryBean, flowNamePrefix, entry.getValue());
 						}
 						registerComponent(pollingChannelAdapterFactoryBean, id, flowBeanName);
 						targetIntegrationComponents.put(pollingChannelAdapterFactoryBean, id);
@@ -238,7 +236,7 @@ else if (component instanceof AnnotationGatewayProxyFactoryBean) {
 						targetIntegrationComponents.put(component, gatewayId);
 					}
 					else {
-						String generatedBeanName = generateBeanName(component, entry.getValue());
+						String generatedBeanName = generateBeanName(component, flowNamePrefix, entry.getValue());
 						registerComponent(component, generatedBeanName, flowBeanName);
 						targetIntegrationComponents.put(component, generatedBeanName);
 					}
@@ -335,19 +333,19 @@ private void registerComponent(Object component, String beanName, String parentN
 		this.beanFactory.getBean(beanName);
 	}
 
-	private String generateBeanName(Object instance) {
-		return generateBeanName(instance, null);
+	private String generateBeanName(Object instance, String prefix) {
+		return generateBeanName(instance, prefix, null);
 	}
 
-	private String generateBeanName(Object instance, String fallbackId) {
+	private String generateBeanName(Object instance, String prefix, String fallbackId) {
 		if (instance instanceof NamedComponent && ((NamedComponent) instance).getComponentName() != null) {
 			return ((NamedComponent) instance).getComponentName();
 		}
 		else if (fallbackId != null) {
 			return fallbackId;
 		}
 
-		String generatedBeanName = instance.getClass().getName();
+		String generatedBeanName = prefix + instance.getClass().getName();
 		String id = generatedBeanName;
 		int counter = -1;
 		while (counter == -1 || this.beanFactory.containsBean(id)) {

spring-integration-core/src/main/java/org/springframework/integration/dsl/context/StandardIntegrationFlowContext.java

@@ -20,6 +20,9 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
@@ -46,7 +49,9 @@
  */
 public final class StandardIntegrationFlowContext implements IntegrationFlowContext, BeanFactoryAware {
 
-	private final Map<String, IntegrationFlowRegistration> registry = new HashMap<>();
+	private final Map<String, IntegrationFlowRegistration> registry = new ConcurrentHashMap<>();
+
+	private final Lock registerFlowsLock = new ReentrantLock();
 
 	private ConfigurableListableBeanFactory beanFactory;
 
@@ -76,17 +81,29 @@ public StandardIntegrationFlowRegistrationBuilder registration(IntegrationFlow i
 	private void register(StandardIntegrationFlowRegistrationBuilder builder) {
 		IntegrationFlow integrationFlow = builder.integrationFlowRegistration.getIntegrationFlow();
 		String flowId = builder.integrationFlowRegistration.getId();
-		if (flowId == null) {
-			flowId = generateBeanName(integrationFlow, null);
-			builder.id(flowId);
+		Lock registerBeanLock = null;
+		try {
+			if (flowId == null) {
+				registerBeanLock = this.registerFlowsLock;
+				registerBeanLock.lock();
+				flowId = generateBeanName(integrationFlow, null);
+				builder.id(flowId);
+			}
+			else if (this.registry.containsKey(flowId)) {
+				throw new IllegalArgumentException("An IntegrationFlow '" + this.registry.get(flowId) +
+						"' with flowId '" + flowId + "' is already registered.\n" +
+						"An existing IntegrationFlowRegistration must be destroyed before overriding.");
+			}
+
+			integrationFlow = (IntegrationFlow) registerBean(integrationFlow, flowId, null);
 		}
-		else if (this.registry.containsKey(flowId)) {
-			throw new IllegalArgumentException("An IntegrationFlow '" + this.registry.get(flowId) +
-					"' with flowId '" + flowId + "' is already registered.\n" +
-					"An existing IntegrationFlowRegistration must be destroyed before overriding.");
+		finally {
+			if (registerBeanLock != null) {
+				registerBeanLock.unlock();
+			}
 		}
-		IntegrationFlow theFlow = (IntegrationFlow) registerBean(integrationFlow, flowId, null);
-		builder.integrationFlowRegistration.setIntegrationFlow(theFlow);
+
+		builder.integrationFlowRegistration.setIntegrationFlow(integrationFlow);
 
 		final String theFlowId = flowId;
 		builder.additionalBeans.forEach((key, value) -> registerBean(key, value, theFlowId));
@@ -133,19 +150,26 @@ public IntegrationFlowRegistration getRegistrationById(String flowId) {
 	 * @param flowId the bean name to destroy from
 	 */
 	@Override
-	public synchronized void remove(String flowId) {
+	public void remove(String flowId) {
 		if (this.registry.containsKey(flowId)) {
 			IntegrationFlowRegistration flowRegistration = this.registry.remove(flowId);
 			flowRegistration.stop();
 
-			Arrays.stream(this.beanFactory.getDependentBeans(flowId))
-					.forEach(((BeanDefinitionRegistry) this.beanFactory)::removeBeanDefinition);
+			BeanDefinitionRegistry beanDefinitionRegistry = (BeanDefinitionRegistry) this.beanFactory;
 
-			((BeanDefinitionRegistry) this.beanFactory).removeBeanDefinition(flowId);
+			Arrays.stream(this.beanFactory.getDependentBeans(flowId))
+					.forEach(beanName -> {
+						beanDefinitionRegistry.removeBeanDefinition(beanName);
+						// TODO until https://jira.spring.io/browse/SPR-16837
+						Arrays.asList(beanDefinitionRegistry.getAliases(beanName))
+								.forEach(beanDefinitionRegistry::removeAlias);
+					});
+
+			beanDefinitionRegistry.removeBeanDefinition(flowId);
 		}
 		else {
-			throw new IllegalStateException("Only manually registered IntegrationFlows can be removed. "
-					+ "But [" + flowId + "] ins't one of them.");
+			throw new IllegalStateException("An IntegrationFlow with the id "
+					+ "[" + flowId + "] doesn't exist in the registry.");
 		}
 	}