Java tutorial
/* * 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. * 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.messaging; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.util.Collection; import java.util.Collections; import java.util.HashMap; import java.util.HashSet; import java.util.Map; import java.util.Set; import java.util.UUID; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; import org.springframework.lang.Nullable; import org.springframework.util.AlternativeJdkIdGenerator; import org.springframework.util.IdGenerator; /** * The headers for a {@link Message}. * * <p><b>IMPORTANT</b>: This class is immutable. Any mutating operation such as * {@code put(..)}, {@code putAll(..)} and others will throw * {@link UnsupportedOperationException}. * <p>Subclasses do have access to the raw headers, however, via {@link #getRawHeaders()}. * * <p>One way to create message headers is to use the * {@link org.springframework.messaging.support.MessageBuilder MessageBuilder}: * <pre class="code"> * MessageBuilder.withPayload("foo").setHeader("key1", "value1").setHeader("key2", "value2"); * </pre> * * A second option is to create {@link org.springframework.messaging.support.GenericMessage} * passing a payload as {@link Object} and headers as a {@link Map java.util.Map}: * <pre class="code"> * Map headers = new HashMap(); * headers.put("key1", "value1"); * headers.put("key2", "value2"); * new GenericMessage("foo", headers); * </pre> * * A third option is to use {@link org.springframework.messaging.support.MessageHeaderAccessor} * or one of its subclasses to create specific categories of headers. * * @author Arjen Poutsma * @author Mark Fisher * @author Gary Russell * @author Juergen Hoeller * @since 4.0 * @see org.springframework.messaging.support.MessageBuilder * @see org.springframework.messaging.support.MessageHeaderAccessor */ public class MessageHeaders implements Map<String, Object>, Serializable { /** * UUID for none. */ public static final UUID ID_VALUE_NONE = new UUID(0, 0); /** * The key for the Message ID. This is an automatically generated UUID and * should never be explicitly set in the header map <b>except</b> in the * case of Message deserialization where the serialized Message's generated * UUID is being restored. */ public static final String ID = "id"; /** * The key for the message timestamp. */ public static final String TIMESTAMP = "timestamp"; /** * The key for the message content type. */ public static final String CONTENT_TYPE = "contentType"; /** * The key for the message reply channel. */ public static final String REPLY_CHANNEL = "replyChannel"; /** * The key for the message error channel. */ public static final String ERROR_CHANNEL = "errorChannel"; private static final long serialVersionUID = 7035068984263400920L; private static final Log logger = LogFactory.getLog(MessageHeaders.class); private static final IdGenerator defaultIdGenerator = new AlternativeJdkIdGenerator(); @Nullable private static volatile IdGenerator idGenerator; private final Map<String, Object> headers; /** * Construct a {@link MessageHeaders} with the given headers. An {@link #ID} and * {@link #TIMESTAMP} headers will also be added, overriding any existing values. * @param headers a map with headers to add */ public MessageHeaders(@Nullable Map<String, Object> headers) { this(headers, null, null); } /** * Constructor providing control over the ID and TIMESTAMP header values. * @param headers a map with headers to add * @param id the {@link #ID} header value * @param timestamp the {@link #TIMESTAMP} header value */ protected MessageHeaders(@Nullable Map<String, Object> headers, @Nullable UUID id, @Nullable Long timestamp) { this.headers = (headers != null ? new HashMap<>(headers) : new HashMap<>()); if (id == null) { this.headers.put(ID, getIdGenerator().generateId()); } else if (id == ID_VALUE_NONE) { this.headers.remove(ID); } else { this.headers.put(ID, id); } if (timestamp == null) { this.headers.put(TIMESTAMP, System.currentTimeMillis()); } else if (timestamp < 0) { this.headers.remove(TIMESTAMP); } else { this.headers.put(TIMESTAMP, timestamp); } } /** * Copy constructor which allows for ignoring certain entries. * Used for serialization without non-serializable entries. * @param original the MessageHeaders to copy * @param keysToIgnore the keys of the entries to ignore */ private MessageHeaders(MessageHeaders original, Set<String> keysToIgnore) { this.headers = new HashMap<>(original.headers.size()); original.headers.forEach((key, value) -> { if (!keysToIgnore.contains(key)) { this.headers.put(key, value); } }); } protected Map<String, Object> getRawHeaders() { return this.headers; } protected static IdGenerator getIdGenerator() { IdGenerator generator = idGenerator; return (generator != null ? generator : defaultIdGenerator); } @Nullable public UUID getId() { return get(ID, UUID.class); } @Nullable public Long getTimestamp() { return get(TIMESTAMP, Long.class); } @Nullable public Object getReplyChannel() { return get(REPLY_CHANNEL); } @Nullable public Object getErrorChannel() { return get(ERROR_CHANNEL); } @SuppressWarnings("unchecked") @Nullable public <T> T get(Object key, Class<T> type) { Object value = this.headers.get(key); if (value == null) { return null; } if (!type.isAssignableFrom(value.getClass())) { throw new IllegalArgumentException("Incorrect type specified for header '" + key + "'. Expected [" + type + "] but actual type is [" + value.getClass() + "]"); } return (T) value; } // Delegating Map implementation public boolean containsKey(Object key) { return this.headers.containsKey(key); } public boolean containsValue(Object value) { return this.headers.containsValue(value); } public Set<Map.Entry<String, Object>> entrySet() { return Collections.unmodifiableMap(this.headers).entrySet(); } @Nullable public Object get(Object key) { return this.headers.get(key); } public boolean isEmpty() { return this.headers.isEmpty(); } public Set<String> keySet() { return Collections.unmodifiableSet(this.headers.keySet()); } public int size() { return this.headers.size(); } public Collection<Object> values() { return Collections.unmodifiableCollection(this.headers.values()); } // Unsupported Map operations /** * Since MessageHeaders are immutable, the call to this method * will result in {@link UnsupportedOperationException}. */ public Object put(String key, Object value) { throw new UnsupportedOperationException("MessageHeaders is immutable"); } /** * Since MessageHeaders are immutable, the call to this method * will result in {@link UnsupportedOperationException}. */ public void putAll(Map<? extends String, ? extends Object> map) { throw new UnsupportedOperationException("MessageHeaders is immutable"); } /** * Since MessageHeaders are immutable, the call to this method * will result in {@link UnsupportedOperationException}. */ public Object remove(Object key) { throw new UnsupportedOperationException("MessageHeaders is immutable"); } /** * Since MessageHeaders are immutable, the call to this method * will result in {@link UnsupportedOperationException}. */ public void clear() { throw new UnsupportedOperationException("MessageHeaders is immutable"); } // Serialization methods private void writeObject(ObjectOutputStream out) throws IOException { Set<String> keysToIgnore = new HashSet<>(); this.headers.forEach((key, value) -> { if (!(value instanceof Serializable)) { keysToIgnore.add(key); } }); if (keysToIgnore.isEmpty()) { // All entries are serializable -> serialize the regular MessageHeaders instance out.defaultWriteObject(); } else { // Some non-serializable entries -> serialize a temporary MessageHeaders copy if (logger.isDebugEnabled()) { logger.debug("Ignoring non-serializable message headers: " + keysToIgnore); } out.writeObject(new MessageHeaders(this, keysToIgnore)); } } private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); } // equals, hashCode, toString @Override public boolean equals(@Nullable Object other) { return (this == other || (other instanceof MessageHeaders && this.headers.equals(((MessageHeaders) other).headers))); } @Override public int hashCode() { return this.headers.hashCode(); } @Override public String toString() { return this.headers.toString(); } }