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

最新推荐文章于 2022-08-12 17:50:07 发布

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探讨了JDK 1.8中HashMap如何通过重写Serializable接口的writeObject和readObject方法实现序列化与反序列化，避免使用transient关键字导致的数据丢失问题。

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 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。

private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
int buckets = capacity();
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
s.writeInt(buckets);
s.writeInt(size);
internalWriteEntries(s);
}
/**
* Reconstitute the {@code HashMap} instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException {
// Read in the threshold (ignored), loadfactor, and any hidden stuff
s.defaultReadObject();
reinitialize();
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
s.readInt(); // Read and ignore number of buckets
int mappings = s.readInt(); // Read number of mappings (size)
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);
else if (mappings > 0) { // (if zero, use defaults)
// Size the table using given load factor only if within
// range of 0.25...4.0
float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);
float fc = (float)mappings / lf + 1.0f;
int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?
DEFAULT_INITIAL_CAPACITY :
(fc >= MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY :
tableSizeFor((int)fc));
float ft = (float)cap * lf;
threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?
(int)ft : Integer.MAX_VALUE);
// Check Map.Entry[].class since it's the nearest public type to
// what we're actually creating.
SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap);
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] tab = (Node<K,V>[])new Node[cap];
table = tab;
// Read the keys and values, and put the mappings in the HashMap
for (int i = 0; i < mappings; i++) {
@SuppressWarnings("unchecked")
K key = (K) s.readObject();
@SuppressWarnings("unchecked")
V value = (V) s.readObject();
putVal(hash(key), key, value, false, false);
}
}
}