GH-723: Add AmqpResourceNotAvailableException (#737)

* GH-723: Add AmqpResourceNotAvailableException

Fixes spring-projects/spring-amqp#723

To avoid an `NPE` when connection returns `null` for the
`createChannel()` in case of `channelMax` is reached, throw newly
introduced `AmqpResourceNotAvailableException`.
This exception can be used in the `RetryPolicy` to retry the original
operation after some back-off - the channel permit may be released in
between

**Cherry-pick to 2.0.x and 1.7.x**

* * Fix `AbstractConnectionFactoryTests` for proper mock
* Fix `amqp.adoc` according PR comments

spring-amqp/src/main/java/org/springframework/amqp/AmqpResourceNotAvailableException.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2018 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
+ *
+ *      http://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.amqp;
+
+/**
+ * The {@link AmqpException} thrown when some resource can't be accessed.
+ * For example when {@code channelMax} limit is reached and connect can't
+ * create a new channel at the moment.
+ *
+ * @author Artem Bilan
+ *
+ * @since 1.7.7
+ */
+public class AmqpResourceNotAvailableException extends AmqpException {
+
+	public AmqpResourceNotAvailableException(String message) {
+		super(message);
+	}
+
+	public AmqpResourceNotAvailableException(Throwable cause) {
+		super(cause);
+	}
+
+	public AmqpResourceNotAvailableException(String message, Throwable cause) {
+		super(message, cause);
+	}
+
+}

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/connection/SimpleConnection.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -19,6 +19,7 @@ package org.springframework.amqp.rabbit.connection;
 import java.io.IOException;
 import java.net.InetAddress;
 
+import org.springframework.amqp.AmqpResourceNotAvailableException;
 import org.springframework.amqp.rabbit.support.RabbitExceptionTranslator;
 import org.springframework.util.ObjectUtils;
 
@@ -53,6 +54,9 @@ public class SimpleConnection implements Connection, NetworkConnection {
 	public Channel createChannel(boolean transactional) {
 		try {
 			Channel channel = this.delegate.createChannel();
+			if (channel == null) {
+				throw new AmqpResourceNotAvailableException("The channelMax limit is reached. Try later.");
+			}
 			if (transactional) {
 				// Just created so we want to start the transaction
 				channel.txSelect();

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/connection/AbstractConnectionFactoryTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2016 the original author or authors.
+ * Copyright 2010-2018 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.
@@ -46,6 +46,7 @@ import org.springframework.amqp.utils.test.TestUtils;
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.scheduling.concurrent.CustomizableThreadFactory;
 
+import com.rabbitmq.client.Channel;
 import com.rabbitmq.client.ConnectionFactory;
 
 /**
@@ -176,6 +177,7 @@ public abstract class AbstractConnectionFactoryTests {
 				.thenReturn(mockConnection1, mockConnection2);
 		// simulate a dead connection
 		when(mockConnection1.isOpen()).thenReturn(false);
+		when(mockConnection2.createChannel()).thenReturn(mock(Channel.class));
 
 		AbstractConnectionFactory connectionFactory = createConnectionFactory(mockConnectionFactory);
 

spring-rabbit/src/test/java/org/springframework/amqp/rabbit/connection/CachingConnectionFactoryIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -60,6 +60,7 @@ import org.junit.rules.ExpectedException;
 import org.springframework.amqp.AmqpAuthenticationException;
 import org.springframework.amqp.AmqpException;
 import org.springframework.amqp.AmqpIOException;
+import org.springframework.amqp.AmqpResourceNotAvailableException;
 import org.springframework.amqp.AmqpTimeoutException;
 import org.springframework.amqp.core.Queue;
 import org.springframework.amqp.rabbit.connection.CachingConnectionFactory.CacheMode;
@@ -87,6 +88,7 @@ import com.rabbitmq.client.impl.recovery.AutorecoveringConnection;
  * @author Gunnar Hillert
  * @author Gary Russell
  * @author Artem Bilan
+ *
  * @since 1.0
  *
  */
@@ -568,6 +570,14 @@ public class CachingConnectionFactoryIntegrationTests {
 		factory.destroy();
 	}
 
+	@Test(expected = AmqpResourceNotAvailableException.class)
+	public void testChannelMax() {
+		this.connectionFactory.getRabbitConnectionFactory().setRequestedChannelMax(1);
+		Connection connection = this.connectionFactory.createConnection();
+		connection.createChannel(true);
+		connection.createChannel(false);
+	}
+
 	private Log spyOnLogger(CachingConnectionFactory connectionFactory2) {
 		DirectFieldAccessor dfa = new DirectFieldAccessor(connectionFactory2);
 		Log logger = spy((Log) dfa.getPropertyValue("logger"));

src/reference/asciidoc/amqp.adoc

@@ -361,6 +361,9 @@ The `ConnectionFactory` argument can be used to distinguish target connection na
 By default a `beanName` of the `AbstractConnectionFactory` and an internal counter are used to generate `connection_name`.
 The `<rabbit:connection-factory>` namespace component is also supplied with the `connection-name-strategy` attribute.
 
+Starting with _version 1.7.7_, an `AmqpResourceNotAvailableException` is provided, which is thrown now when `SimpleConnection.createChannel()` can't create a `Channel`, for example, because the `channelMax` limit is reached and there are no available channels in the cache.
+This exception can be used in the `RetryPolicy` to recover the operation after some back-off.
+
 [[connection-factory]]
 ===== Configuring the Underlying Client Connection Factory