关于jdk1.8容器hashmap中的变量table加了transient修饰后如何序列化。Serializable

本是古典何须时尚 2022-02-26 01:56 136阅读 0赞

# 关于jdk1.8容器hashmap中的变量table加了transient修饰后如何序列化。Serializable #

2018年03月12日 11:13:24 [qq\_35516657][qq_35516657] 阅读数：462

众所周知，transient修饰符的作用是使该变量在序列化的时候不会被储存。

transient Node<K,V>[] table;

但是hashmap中的变量table是储存了容器中所有的元素，在序列化中不被储存，那么反序列化后hashmap对象中岂不是个空容器？

后来通过细想，table里存的只是引用，就算在序列化中储存到硬盘里，反序列化后table变量里的引用已经没有意义了。

至于hashmap是如何在序列化中储存元素呢？原来是它通过重写Serializable接口中的writeObject方法和readObject方法实现的。下面贴出两个方法的源代码。

1.  `private void writeObject(java.io.ObjectOutputStream s)`
2.  `throws IOException {`
3.  `int buckets = capacity();`
4.  `// Write out the threshold, loadfactor, and any hidden stuff`
5.  `s.defaultWriteObject();`
6.  `s.writeInt(buckets);`
7.  `s.writeInt(size);`
8.  `internalWriteEntries(s);`
9.  `}`
10.  
11. `/**`
12. `* Reconstitute the {@code HashMap} instance from a stream (i.e.,`
13. `* deserialize it).`
14. `*/`
15. `private void readObject(java.io.ObjectInputStream s)`
16. `throws IOException, ClassNotFoundException {`
17. `// Read in the threshold (ignored), loadfactor, and any hidden stuff`
18. `s.defaultReadObject();`
19. `reinitialize();`
20. `if (loadFactor <= 0 || Float.isNaN(loadFactor))`
21. `throw new InvalidObjectException("Illegal load factor: " +`
22. `loadFactor);`
23. `s.readInt(); // Read and ignore number of buckets`
24. `int mappings = s.readInt(); // Read number of mappings (size)`
25. `if (mappings < 0)`
26. `throw new InvalidObjectException("Illegal mappings count: " +`
27. `mappings);`
28. `else if (mappings > 0) { // (if zero, use defaults)`
29. `// Size the table using given load factor only if within`
30. `// range of 0.25...4.0`
31. `float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);`
32. `float fc = (float)mappings / lf + 1.0f;`
33. `int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?`
34. `DEFAULT_INITIAL_CAPACITY :`
35. `(fc >= MAXIMUM_CAPACITY) ?`
36. `MAXIMUM_CAPACITY :`
37. `tableSizeFor((int)fc));`
38. `float ft = (float)cap * lf;`
39. `threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?`
40. `(int)ft : Integer.MAX_VALUE);`
41.  
42. `// Check Map.Entry[].class since it's the nearest public type to`
43. `// what we're actually creating.`
44. `SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap);`
45. `@SuppressWarnings({"rawtypes","unchecked"})`
46. `Node<K,V>[] tab = (Node<K,V>[])new Node[cap];`
47. `table = tab;`
48.  
49. `// Read the keys and values, and put the mappings in the HashMap`
50. `for (int i = 0; i < mappings; i++) {`
51. `@SuppressWarnings("unchecked")`
52. `K key = (K) s.readObject();`
53. `@SuppressWarnings("unchecked")`
54. `V value = (V) s.readObject();`
55. `putVal(hash(key), key, value, false, false);`
56. `}`
57. `}`
58. `}`

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