Polishing Javadoc

See gh-25821

spring-messaging/src/main/java/org/springframework/messaging/support/MessageHeaderAccessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -39,76 +39,62 @@ import org.springframework.util.PatternMatchUtils;
 import org.springframework.util.StringUtils;
 
 /**
- * A base for classes providing strongly typed getters and setters as well as
+ * Wrapper around {@link MessageHeaders} that provides extra features such as
- * behavior around specific categories of headers (e.g. STOMP headers).
+ * strongly typed accessors for specific headers, the ability to leave headers
- * Supports creating new headers, modifying existing headers (when still mutable),
+ * in a {@link Message} mutable, and the option to suppress automatic generation
- * or copying and modifying existing headers.
+ * of {@link MessageHeaders#ID id} and {@link MessageHeaders#TIMESTAMP
- *
+ * timesteamp} headers. Sub-classes such as {@link NativeMessageHeaderAccessor}
- * <p>The method {@link #getMessageHeaders()} provides access to the underlying,
+ * and others provide support for managing processing vs external source headers
- * fully-prepared {@link MessageHeaders} that can then be used as-is (i.e.
+ * as well as protocol specific headers.
- * without copying) to create a single message as follows:
  *
+ * <p>Below is a workflow to initialize headers via {@code MessageHeaderAccessor},
+ * or one of its sub-classes, then create a {@link Message}, and then re-obtain
+ * the accessor possibly from a different component:
  * <pre class="code">
+ * // Create a message with headers
  * MessageHeaderAccessor accessor = new MessageHeaderAccessor();
  * accessor.setHeader("foo", "bar");
- * Message message = MessageBuilder.createMessage("payload", accessor.getMessageHeaders());
+ * MessageHeaders headers = accessor.getMessageHeaders();
+ * Message message = MessageBuilder.createMessage("payload", headers);
+ *
+ * // Later on
+ * MessageHeaderAccessor accessor = MessageHeaderAccessor.getAccessor(message);
+ * Assert.notNull(accessor, "No MessageHeaderAccessor");
  * </pre>
  *
- * <p>After the above, by default the {@code MessageHeaderAccessor} becomes
+ * <p>In order for the above to work, all participating components must use
- * immutable. However it is possible to leave it mutable for further initialization
+ * {@code MessageHeaders} to create, access, or modify headers, or otherwise
- * in the same thread, for example:
+ * {@link MessageHeaderAccessor#getAccessor(Message, Class)} will return null.
- *
+ * Below is a workflow that shows how headers are created and left mutable,
+ * then modified possibly by a different component, and finally made immutable
+ * perhaps before the possibility of being accessed on a different thread:
  * <pre class="code">
+ * // Create a message with mutable headers
  * MessageHeaderAccessor accessor = new MessageHeaderAccessor();
  * accessor.setHeader("foo", "bar");
  * accessor.setLeaveMutable(true);
- * Message message = MessageBuilder.createMessage("payload", accessor.getMessageHeaders());
+ * MessageHeaders headers = accessor.getMessageHeaders();
- *
+ * Message message = MessageBuilder.createMessage("payload", headers);
- * // later on in the same thread...
  *
+ * // Later on
  * MessageHeaderAccessor accessor = MessageHeaderAccessor.getAccessor(message);
+ * if (accessor.isMutable()) {
+ *     // It's mutable, just change the headers
  *     accessor.setHeader("bar", "baz");
+ * }
+ * else {
+ *     // It's not, so get a mutable copy, change and re-create
+ *     accessor = MessageHeaderAccessor.getMutableAccessor(message);
+ *     accessor.setHeader("bar", "baz");
+ *     accessor.setLeaveMutable(true); // leave mutable again or not?
+ *     message = MessageBuilder.createMessage(message.getPayload(), accessor);
+ * }
+ *
+ * // Make the accessor immutable
+ * MessageHeaderAccessor accessor = MessageHeaderAccessor.getAccessor(message);
  * accessor.setImmutable();
  * </pre>
  *
- * <p>The method {@link #toMap()} returns a copy of the underlying headers. It can
- * be used to prepare multiple messages from the same {@code MessageHeaderAccessor}
- * instance:
- * <pre class="code">
- * MessageHeaderAccessor accessor = new MessageHeaderAccessor();
- * MessageBuilder builder = MessageBuilder.withPayload("payload").setHeaders(accessor);
- *
- * accessor.setHeader("foo", "bar1");
- * Message message1 = builder.build();
- *
- * accessor.setHeader("foo", "bar2");
- * Message message2 = builder.build();
- *
- * accessor.setHeader("foo", "bar3");
- * Message  message3 = builder.build();
- * </pre>
- *
- * <p>However note that with the above style, the header accessor is shared and
- * cannot be re-obtained later on. Alternatively it is also possible to create
- * one {@code MessageHeaderAccessor} per message:
- *
- * <pre class="code">
- * MessageHeaderAccessor accessor1 = new MessageHeaderAccessor();
- * accessor.set("foo", "bar1");
- * Message message1 = MessageBuilder.createMessage("payload", accessor1.getMessageHeaders());
- *
- * MessageHeaderAccessor accessor2 = new MessageHeaderAccessor();
- * accessor.set("foo", "bar2");
- * Message message2 = MessageBuilder.createMessage("payload", accessor2.getMessageHeaders());
- *
- * MessageHeaderAccessor accessor3 = new MessageHeaderAccessor();
- * accessor.set("foo", "bar3");
- * Message message3 = MessageBuilder.createMessage("payload", accessor3.getMessageHeaders());
- * </pre>
- *
- * <p>Note that the above examples aim to demonstrate the general idea of using
- * header accessors. The most likely usage however is through subclasses.
- *
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
  * @since 4.0
@@ -568,6 +554,21 @@ public class MessageHeaderAccessor {
 
 	// Static factory methods
 
+	/**
+	 * Return the original {@code MessageHeaderAccessor} used to create the headers
+	 * of the given {@code Message}, or {@code null} if that's not available or if
+	 * its type does not match the required type.
+	 * <p>This is for cases where the existence of an accessor is strongly expected
+	 * (followed up with an assertion) or where an accessor will be created otherwise.
+	 * @param message the message to get an accessor for
+	 * @return an accessor instance of the specified type, or {@code null} if none
+	 * @since 5.1.19
+	 */
+	@Nullable
+	public static MessageHeaderAccessor getAccessor(Message<?> message) {
+		return getAccessor(message.getHeaders(), null);
+	}
+
 	/**
 	 * Return the original {@code MessageHeaderAccessor} used to create the headers
 	 * of the given {@code Message}, or {@code null} if that's not available or if
@@ -627,6 +628,11 @@ public class MessageHeaderAccessor {
 	}
 
 
+	/**
+	 * Extension of {@link MessageHeaders} that helps to preserve the link to
+	 * the outer {@link MessageHeaderAccessor} instance that created it as well
+	 * as keeps track of whether headers are still mutable.
+	 */
 	@SuppressWarnings("serial")
 	private class MutableMessageHeaders extends MessageHeaders {
 

spring-messaging/src/main/java/org/springframework/messaging/support/NativeMessageHeaderAccessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -30,40 +30,34 @@ import org.springframework.util.MultiValueMap;
 import org.springframework.util.ObjectUtils;
 
 /**
- * An extension of {@link MessageHeaderAccessor} that also stores and provides read/write
+ * {@link MessageHeaderAccessor} sub-class that supports storage and access of
- * access to message headers from an external source -- e.g. a Spring {@link Message}
+ * headers from an external source such as a message broker. Headers from the
- * created to represent a STOMP message received from a STOMP client or message broker.
+ * external source are kept separate from other headers, in a sub-map under the
- * Native message headers are kept in a {@code Map<String, List<String>>} under the key
+ * key {@link #NATIVE_HEADERS}. This allows separating processing headers from
- * {@link #NATIVE_HEADERS}.
+ * headers that need to be sent to or received from the external source.
  *
- * <p>This class is not intended for direct use but is rather expected to be used
+ * <p>This class is likely to be used through indirectly through a protocol
- * indirectly through protocol-specific sub-classes such as
+ * specific sub-class that also provide factory methods to translate
- * {@link org.springframework.messaging.simp.stomp.StompHeaderAccessor StompHeaderAccessor}.
+ * message headers to an from an external messaging source.
- * Such sub-classes may provide factory methods to translate message headers from
- * an external messaging source (e.g. STOMP) to Spring {@link Message} headers and
- * reversely to translate Spring {@link Message} headers to a message to send to an
- * external source.
  *
  * @author Rossen Stoyanchev
  * @since 4.0
  */
 public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 
-	/**
+	/** The header name used to store native headers. */
-	 * The header name used to store native headers.
-	 */
 	public static final String NATIVE_HEADERS = "nativeHeaders";
 
 
 	/**
-	 * A protected constructor to create new headers.
+	 * Protected constructor to create a new instance.
 	 */
 	protected NativeMessageHeaderAccessor() {
 		this((Map<String, List<String>>) null);
 	}
 
 	/**
-	 * A protected constructor to create new headers.
+	 * Protected constructor to create an instance with the given native headers.
 	 * @param nativeHeaders native headers to create the message with (may be {@code null})
 	 */
 	protected NativeMessageHeaderAccessor(@Nullable Map<String, List<String>> nativeHeaders) {
@@ -73,7 +67,7 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 	}
 
 	/**
-	 * A protected constructor accepting the headers of an existing message to copy.
+	 * Protected constructor that copies headers from another message.
 	 */
 	protected NativeMessageHeaderAccessor(@Nullable Message<?> message) {
 		super(message);
@@ -88,6 +82,10 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 		}
 	}
 
+
+	/**
+	 * Sub-classes can use this method to access the "native" headers sub-map.
+	 */
 	@SuppressWarnings("unchecked")
 	@Nullable
 	protected Map<String, List<String>> getNativeHeaders() {
@@ -95,7 +93,7 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 	}
 
 	/**
-	 * Return a copy of the native header values or an empty map.
+	 * Return a copy of the native headers sub-map, or an empty map.
 	 */
 	public Map<String, List<String>> toNativeHeaderMap() {
 		Map<String, List<String>> map = getNativeHeaders();
@@ -124,8 +122,7 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 	}
 
 	/**
-	 * Return all values for the specified native header.
+	 * Return the values for the specified native header, if present.
-	 * or {@code null} if none.
 	 */
 	@Nullable
 	public List<String> getNativeHeader(String headerName) {
@@ -134,8 +131,7 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 	}
 
 	/**
-	 * Return the first value for the specified native header,
+	 * Return the first value for the specified native header, if present.
-	 * or {@code null} if none.
 	 */
 	@Nullable
 	public String getFirstNativeHeader(String headerName) {
@@ -151,6 +147,8 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 
 	/**
 	 * Set the specified native header value replacing existing values.
+	 * <p>In order for this to work, the accessor must be {@link #isMutable()
+	 * mutable}. See {@link MessageHeaderAccessor} for details.
 	 */
 	public void setNativeHeader(String name, @Nullable String value) {
 		Assert.state(isMutable(), "Already immutable");
@@ -176,6 +174,8 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 
 	/**
 	 * Add the specified native header value to existing values.
+	 * <p>In order for this to work, the accessor must be {@link #isMutable()
+	 * mutable}. See {@link MessageHeaderAccessor} for details.
 	 */
 	public void addNativeHeader(String name, @Nullable String value) {
 		Assert.state(isMutable(), "Already immutable");
@@ -199,6 +199,11 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 		headers.forEach((key, values) -> values.forEach(value -> addNativeHeader(key, value)));
 	}
 
+	/**
+	 * Remove the specified native header value replacing existing values.
+	 * <p>In order for this to work, the accessor must be {@link #isMutable()
+	 * mutable}. See {@link MessageHeaderAccessor} for details.
+	 */
 	@Nullable
 	public List<String> removeNativeHeader(String name) {
 		Assert.state(isMutable(), "Already immutable");

spring-messaging/src/test/java/org/springframework/messaging/support/NativeMessageHeaderAccessorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -112,8 +112,8 @@ public class NativeMessageHeaderAccessorTests {
 				(Map<String, List<String>>) actual.get(NativeMessageHeaderAccessor.NATIVE_HEADERS);
 
 		assertThat(actualNativeHeaders).isNotNull();
-		assertThat(actualNativeHeaders.get("foo")).isEqualTo(Arrays.asList("BAR"));
+		assertThat(actualNativeHeaders.get("foo")).isEqualTo(Collections.singletonList("BAR"));
-		assertThat(actualNativeHeaders.get("bar")).isEqualTo(Arrays.asList("baz"));
+		assertThat(actualNativeHeaders.get("bar")).isEqualTo(Collections.singletonList("baz"));
 	}
 
 	@Test
@@ -124,7 +124,7 @@ public class NativeMessageHeaderAccessorTests {
 		NativeMessageHeaderAccessor headers = new NativeMessageHeaderAccessor(nativeHeaders);
 		headers.setNativeHeader("foo", "baz");
 
-		assertThat(headers.getNativeHeader("foo")).isEqualTo(Arrays.asList("baz"));
+		assertThat(headers.getNativeHeader("foo")).isEqualTo(Collections.singletonList("baz"));
 	}
 
 	@Test
@@ -143,7 +143,7 @@ public class NativeMessageHeaderAccessorTests {
 		NativeMessageHeaderAccessor headerAccessor = new NativeMessageHeaderAccessor();
 		headerAccessor.setNativeHeader("foo", "baz");
 
-		assertThat(headerAccessor.getNativeHeader("foo")).isEqualTo(Arrays.asList("baz"));
+		assertThat(headerAccessor.getNativeHeader("foo")).isEqualTo(Collections.singletonList("baz"));
 	}
 
 	@Test
@@ -161,8 +161,8 @@ public class NativeMessageHeaderAccessorTests {
 		headerAccessor.setNativeHeader("foo", "bar");
 		headerAccessor.setImmutable();
 
-		assertThatIllegalStateException().isThrownBy(() ->
+		assertThatIllegalStateException()
-				headerAccessor.setNativeHeader("foo", "baz"))
+				.isThrownBy(() -> headerAccessor.setNativeHeader("foo", "baz"))
 				.withMessageContaining("Already immutable");
 	}
 
@@ -185,7 +185,7 @@ public class NativeMessageHeaderAccessorTests {
 		NativeMessageHeaderAccessor headers = new NativeMessageHeaderAccessor(nativeHeaders);
 		headers.addNativeHeader("foo", null);
 
-		assertThat(headers.getNativeHeader("foo")).isEqualTo(Arrays.asList("bar"));
+		assertThat(headers.getNativeHeader("foo")).isEqualTo(Collections.singletonList("bar"));
 	}
 
 	@Test
@@ -193,7 +193,7 @@ public class NativeMessageHeaderAccessorTests {
 		NativeMessageHeaderAccessor headerAccessor = new NativeMessageHeaderAccessor();
 		headerAccessor.addNativeHeader("foo", "bar");
 
-		assertThat(headerAccessor.getNativeHeader("foo")).isEqualTo(Arrays.asList("bar"));
+		assertThat(headerAccessor.getNativeHeader("foo")).isEqualTo(Collections.singletonList("bar"));
 	}
 
 	@Test
@@ -211,8 +211,8 @@ public class NativeMessageHeaderAccessorTests {
 		headerAccessor.addNativeHeader("foo", "bar");
 		headerAccessor.setImmutable();
 
-		assertThatIllegalStateException().isThrownBy(() ->
+		assertThatIllegalStateException()
-				headerAccessor.addNativeHeader("foo", "baz"))
+				.isThrownBy(() -> headerAccessor.addNativeHeader("foo", "baz"))
 				.withMessageContaining("Already immutable");
 	}