java源码解析---LinkedHashMap
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1 package explain; 2 3 import java.util.function.Consumer; 4 import java.util.AbstractCollection; 5 import java.util.AbstractSet; 6 import java.util.HashMap; 7 import java.util.Iterator; 8 import java.util.Map; 9 import java.util.Set; 10 import java.util.function.BiConsumer; 11 import java.util.function.BiFunction; 12 import java.io.IOException; 13 14 /** 15 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface, 16 * with predictable iteration order. This implementation differs from 17 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through 18 * all of its entries. This linked list defines the iteration ordering, 19 * which is normally the order in which keys were inserted into the map 20 * (<i>insertion-order</i>). Note that insertion order is not affected 21 * if a key is <i>re-inserted</i> into the map. (A key <tt>k</tt> is 22 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when 23 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to 24 * the invocation.) 25 * Map接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与HashMap的不同之处在于， 26 后者维护着一个运行于所有条目的双重链接列表。 27 此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。 28 注意，如果在映射中重新插入键，则插入顺序不受影响。会按照第一次插入的顺序，再次插入到之前的位置 29 （如果在调用m.put(k,v) 前 m.containsKey(k) 返回了true，则调用时会将键k重新插入到映射m中） 30 * <p>This implementation spares its clients from the unspecified, generally 31 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}), 32 * without incurring the increased cost associated with {@link TreeMap}. It 33 * can be used to produce a copy of a map that has the same order as the 34 * original, regardless of the original map\'s implementation: 35 此实现可以让客户避免未指定的、由 HashMap（及 Hashtable）所提供的通常为杂乱无章的排序工作，同时无需增加与 TreeMap 相关的成本。
使用它可以生成一个与原来顺序相同的映射副本，而与原映射的实现无关： 36 * <pre> 37 * void foo(Map m) { 38 * Map copy = new LinkedHashMap(m); 39 * ... 40 * } 41 * </pre> 42 * This technique is particularly useful if a module takes a map on input, 43 * copies it, and later returns results whose order is determined by that of 44 * the copy. (Clients generally appreciate having things returned in the same 45 * order they were presented.) 46 * 如果模块通过输入得到一个映射，复制这个映射，然后返回由此副本确定其顺序的结果，这种情况下这项技术特别有用。 47 （客户通常期望返回的内容与其出现的顺序相同。） 原样返回 48 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is 49 * provided to create a linked hash map whose order of iteration is the order 50 * in which its entries were last accessed, from least-recently accessed to 51 * most-recently (<i>access-order</i>). This kind of map is well-suited to 52 * building LRU caches. Invoking the {@code put}, {@code putIfAbsent}, 53 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent}, 54 * {@code computeIfPresent}, or {@code merge} methods results 55 * in an access to the corresponding entry (assuming it exists after the 56 * invocation completes). The {@code replace} methods only result in an access 57 * of the entry if the value is replaced. The {@code putAll} method generates one 58 * entry access for each mapping in the specified map, in the order that 59 * key-value mappings are provided by the specified map\'s entry set iterator. 60 * <i>No other methods generate entry accesses.</i> In particular, operations 61 * on collection-views do <i>not</i> affect the order of iteration of the 62 * backing map. 63 提供特殊的构造方法来创建链接哈希映射，该哈希映射的迭代顺序就是最后访问其条目的顺序， 64 从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建 LRU 缓存。调用put或get 方法将会访问相应的条目（假定调用完成后它还存在）。
putAll 方法以指定映射的条目集迭代器提供的键-值映射关系的顺序，为指定映射的每个映射关系生成一个条目访问。 65 任何其他方法均不生成条目访问。特别是，collection 视图上的操作不 影响底层映射的迭代顺序 66 * 67 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to 68 * impose a policy for removing stale mappings automatically when new mappings 69 * are added to the map. 70 可以重写 removeEldestEntry(Map.Entry) 方法来实施策略，以便在将新映射关系添加到映射时自动移除旧的映射关系。 71 * <p>This class provides all of the optional <tt>Map</tt> operations, and 72 * permits null elements. Like <tt>HashMap</tt>, it provides constant-time 73 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and 74 * <tt>remove</tt>), assuming the hash function disperses elements 75 * properly among the buckets. Performance is likely to be just slightly 76 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the 77 * linked list, with one exception: Iteration over the collection-views 78 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i> 79 * of the map, regardless of its capacity. Iteration over a <tt>HashMap</tt> 80 * is likely to be more expensive, requiring time proportional to its 81 * <i>capacity</i>. 82 *此类提供所有可选的 Map 操作，并且允许 null 元素。与 HashMap 一样， 83 它可以为基本操作（add、contains和remove）提供稳定的性能，假定哈希函数将元素正确分布到桶中。 84 由于增加了维护链接列表的开支，其性能很可能比 HashMap 稍逊一筹，不过这一点例外：LinkedHashMap 的 collection 视图迭代所需时间与映射的大小成比例。
HashMap 迭代时间很可能开支较大，因为它所需要的时间与其容量 成比例。 85 * <p>A linked hash map has two parameters that affect its performance: 86 * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely 87 * as for <tt>HashMap</tt>. Note, however, that the penalty for choosing an 88 * excessively high value for initial capacity is less severe for this class 89 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected 90 * by capacity. 91 *链接的哈希映射具有两个影响其性能的参数：初始容量和加载因子。它们的定义与 HashMap 极其相似。要注意，为初始容量选择非常高的值对此类的影响比对 HashMap 要小，
因为此类的迭代时间不受容量的影响。 92 * <p><strong>Note that this implementation is not synchronized.</strong> 93 * If multiple threads access a linked hash map concurrently, and at least 94 * one of the threads modifies the map structurally, it <em>must</em> be 95 * synchronized externally. This is typically accomplished by 96 * synchronizing on some object that naturally encapsulates the map. 97 * 98 * If no such object exists, the map should be "wrapped" using the 99 * {@link Collections#synchronizedMap Collections.synchronizedMap} 100 * method. This is best done at creation time, to prevent accidental 101 * unsynchronized access to the map:<pre> 102 * Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre> 103 * 注意，此实现不是同步的。如果多个线程同时访问链接的哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须 保持外部同步。
这一般通过对自然封装该映射的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedMap 方法来“包装”该映射。最好在创建时完成这一操作，
以防止对映射的意外的非同步访问： 104 * A structural modification is any operation that adds or deletes one or more 105 * mappings or, in the case of access-ordered linked hash maps, affects 106 * iteration order. In insertion-ordered linked hash maps, merely changing 107 * the value associated with a key that is already contained in the map is not 108 * a structural modification. <strong>In access-ordered linked hash maps, 109 * merely querying the map with <tt>get</tt> is a structural modification. 110 * </strong>) 111 结构修改是指添加或删除一个或多个映射关系，或者在按访问顺序链接的哈希映射中影响迭代顺序的任何操作。 112 在按插入顺序链接的哈希映射中，仅更改与映射中已包含键关联的值不是结构修改。在按访问顺序链接的哈希映射中，仅利用 get 查询映射是结构修改。）1.6和1.8有区别 113 *当accessOrder为true时，在迭代器方式遍历map时，不允许使用get操作，因为在迭代器模式中不允许修改集合的结构，一般情况下get操作是只读的，
但是本模式下，get操作需要修改map的链表结构，以将最近访问的元素放置到链表末尾。 114 115 * <p>The iterators returned by the <tt>iterator</tt> method of the collections 116 * returned by all of this class\'s collection view methods are 117 * <em>fail-fast</em>: if the map is structurally modified at any time after 118 * the iterator is created, in any way except through the iterator\'s own 119 * <tt>remove</tt> method, the iterator will throw a {@link 120 * ConcurrentModificationException}. Thus, in the face of concurrent 121 * modification, the iterator fails quickly and cleanly, rather than risking 122 * arbitrary, non-deterministic behavior at an undetermined time in the future. 123 *Collection（由此类的所有 collection 视图方法所返回）的 iterator 方法返回的迭代器都是快速失败 的：在迭代器创建之后，如果从结构上对映射进行修改，
除非通过迭代器自身的 remove方法，其他任何时间任何方式的修改，迭代器都将抛出ConcurrentModificationException。 124 因此，面对并发的修改，迭代器很快就会完全失败， 125 而不冒将来不确定的时间任意发生不确定行为的风险。 126 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed 127 * as it is, generally speaking, impossible to make any hard guarantees in the 128 * presence of unsynchronized concurrent modification. Fail-fast iterators 129 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 130 * Therefore, it would be wrong to write a program that depended on this 131 * exception for its correctness: <i>the fail-fast behavior of iterators 132 * should be used only to detect bugs.</i> 133 *注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。 134 快速失败迭代器会尽最大努力抛出ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的， 135 正确做法是：迭代器的快速失败行为应该仅用于检测程序错误 136 137 * 138 * <p>This class is a member of the 139 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 140 * Java Collections Framework</a>. 141 * 142 * @implNote 143 * The spliterators returned by the spliterator method of the collections 144 * returned by all of this class\'s collection view methods are created from 145 * the iterators of the corresponding collections. 146 * 147 * @param <K> the type of keys maintained by this map 148 * @param <V> the type of mapped values 149 * 150 * @author Josh Bloch 151 * @see Object#hashCode() 152 * @see Collection 153 * @see Map 154 * @see HashMap 155 * @see TreeMap 156 * @see Hashtable 157 * @since 1.4 158 */ 159 160 public class LinkedHashMap<K,V> 161 extends HashMap<K,V> 162 implements Map<K,V> 163 { 164 165 private static final long serialVersionUID = 3801124242820219131L; 166 167 /** 168 * The head of the doubly linked list. 169 * 双向链表的头 170 */ 171 private transient Entry<K,V> header; 172 173 /** 174 * The iteration ordering method for this linked hash map: <tt>true</tt> 175 * for access-order, <tt>false</tt> for insertion-order. 176 * LinkedHashMap 迭代的排序方法 当为True时，表示按访问顺序排序 177 当为false时，表示按插入顺序排序，默认是按插入顺序排序 178 * @serial 179 */ 180 private final boolean accessOrder; 181 182 /** 183 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 184 * with the specified initial capacity and load factor. 185 * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和负载因子，调用父类的构造器构造 186 * @param initialCapacity the initial capacity 187 * @param loadFactor the load factor 188 * @throws IllegalArgumentException if the initial capacity is negative 189 * or the load factor is nonpositive 190 */ 191 public LinkedHashMap(int initialCapacity, float loadFactor) { 192 super(initialCapacity, loadFactor); 193 accessOrder = false; 194 } 195 196 /** 197 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 198 * with the specified initial capacity and a default load factor (0.75). 199 * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和默认的负载因子0.75，调用父类的构造器构造 200 * @param initialCapacity the initial capacity 201 * @throws IllegalArgumentException if the initial capacity is negative 202 */ 203 public LinkedHashMap(int initialCapacity) { 204 super(initialCapacity); 205 accessOrder = false; 206 } 207 208 /** 209 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 210 * with the default initial capacity (16) and load factor (0.75). 211 构造一个空的按插入顺序排序的LinkedHashMap,并指定默认初始化容量16和默认的负载因子0.75，调用父类的构造器构造 212 */ 213 public LinkedHashMap() { 214 super(); 215 accessOrder = false; 216 } 217 218 /** 219 * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with 220 * the same mappings as the specified map. The <tt>LinkedHashMap</tt> 221 * instance is created with a default load factor (0.75) and an initial 222 * capacity sufficient to hold the mappings in the specified map. 223 * 用指定的Map构造一个按插入顺序排序的LinkedHashMap，默认的负载因子0.75和足够容纳给定map的初始化容量 224 * @param m the map whose mappings are to be placed in this map 225 * @throws NullPointerException if the specified map is null 226 */ 227 public LinkedHashMap(Map<? extends K, ? extends V> m) { 228 super(m); 229 accessOrder = false; 230 } 231 232 /** 233 * Constructs an empty <tt>LinkedHashMap</tt> instance with the 234 * specified initial capacity, load factor and ordering mode. 235 * 构造一个空的按插入顺序排序的LinkedHashMap，并指定初始化容量,负载因子和排序方式 236 * @param initialCapacity the initial capacity 237 * @param loadFactor the load factor 238 * @param accessOrder the ordering mode - <tt>true</tt> for 239 * access-order, <tt>false</tt> for insertion-order 240 * @throws IllegalArgumentException if the initial capacity is negative 241 * or the load factor is nonpositive 242 */ 243 public LinkedHashMap(int initialCapacity, 244 float loadFactor, 245 boolean accessOrder) { 246 super(initialCapacity, loadFactor); 247 this.accessOrder = accessOrder; 248 } 249 250 /** 251 * Called by superclass constructors and pseudoconstructors (clone, 252 * readObject) before any entries are inserted into the map. Initializes 253 * the chain. 254 覆写父类的init方法，在父类中都是空方法，在任何键值对被放入map之前在父类的构造器和伪构造器中调用 255 初始化双向链表，初始化一个空的节点，hashcode的值是-1，并不存放在哈希表中，作为双向循环链表的头部， 256 循环的开始节点或者可以认为也是结束的标志 257 */ 258 void init() { 259 header = new Entry<K,V>(-1, null, null, null); 260 header.before = header.after = header; 261 } 262 263 /** 264 * Transfers all entries to new table array. This method is called 265 * by superclass resize. It is overridden for performance, as it is 266 * faster to iterate using our linked list. 267 将所有的键值对复制到新的哈希表中，该方法在父类的扩容被调用 268 该方法被重写是因为他的性能，因为我们用维护的链表遍历速度更快 269 */ 270 void transfer(HashMap.Entry[] newTable) { 271 int newCapacity = newTable.length; 272 for (Entry<K,V> e = header.after; e != header; e = e.after) { 273 int index = indexFor(e.hash, newCapacity); 274 e.next = newTable[index]; 275 newTable[index] = e; 276 } 277 } 278 279 280 /** 281 * Returns <tt>true</tt> if this map maps one or more keys to the 282 * specified value. 283 * 284 * @param value value whose presence in this map is to be tested 285 * @return <tt>true</tt> if this map maps one or more keys to the 286 * specified value 287 */ 288 public boolean containsValue(Object value) { 289 // Overridden to take advantage of faster iterator 290 //重写是为了性能，链表的遍历速度更快 291 if (value==null) { 292 for (Entry e = header.after; e != header; e = e.after) 293 if (e.value==null) 294 return true; 295 } else { 296 for (Entry e = header.after; e != header; e = e.after) 297 if (value.equals(e.value)) 298 return true; 299 } 300 return false; 301 } 302 303 /** 304 * Returns the value to which the specified key is mapped, 305 * or {@code null} if this map contains no mapping for the key. 306 * 根据指定的Map的Key返回value值，如果没有返回null，也可能是key对于的Value就是null 307 * <p>More formally, if this map contains a mapping from a key 308 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 309 * key.equals(k))}, then this method returns {@code v}; otherwise 310 * it returns {@code null}. (There can be at most one such mapping.) 311 * 更正式点说，如果存在这样个map(key==null ? k==null : key.equals(k) 312 * 返回value，否则返回null，最多只有一个这个的键值对 313 * <p>A return value of {@code null} does not <i>necessarily</i> 314 * indicate that the map contains no mapping for the key; it\'s also 315 * possible that the map explicitly maps the key to {@code null}. 316 * The {@link #containsKey containsKey} operation may be used to 317 * distinguish these two cases. 318 并不一定会返回null，如果返回null值，表示不存在指定key的map，或者可能是该key的Value就是null 319 可以通过containsKey操作来判别 320 */ 321 public V get(Object key) { 322 Entry<K,V> e = (Entry<K,V>)getEntry(key); 323 if (e == null) 324 return null; 325 e.recordAccess(this); 326 return e.value; 327 } 328 329 /** 330 * Removes all of the mappings from this map. 331 * The map will be empty after this call returns. 332 删除所有键值对 333 */ 334 public void clear() { 335 super.clear(); 336 header.before = header.after = header; 337 } 338 339 /** 340 * LinkedHashMap entry. 341 LinkedHashMap的键值对对象，继承了HashMap的属性hash，key，value，next，并且新增属性 before和after，前四个属性是为了维护哈希表，before和after属性是为了维护双向循环链表 342 */ 343 private static class Entry<K,V> extends HashMap.Entry<K,V> { 344 // These fields comprise the doubly linked list used for iteration. 345 //用来对双向循环链表进行遍历 346 Entry<K,V> before, after; 347 348 Entry(int hash, K key, V value, HashMap.Entry<K,V> next) { 349 super(hash, key, value, next); 350 } 351 352 /** 353 * Removes this entry from the linked list. 354 从双向链表中移除当前节点 355 */ 356 private void remove() { 357 before.after = after; 358 after.before = before; 359 } 360 361 /** 362 * Inserts this entry before the specified existing entry in the list. 363 在已存在的双向链表中的某个节以上是关于java源码解析---LinkedHashMap的主要内容，如果未能解决你的问题，请参考以下文章