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分享Hashtable还是线程安全的吗
在查看JDK1.6中Hashtable的源码时，发现所有的方法都没有synchronized关键字修饰，而且源码中有如下注释：
请问各位，现在的Hashtable是不是也并不是线程安全的了？为什么不再有synchronized修饰了？在网上也没查到什么，有没有什么相关资料可以给我推荐我阅读一下的，谢谢！
/**
* IMPORTANT NOTE:
* This code was taken from Sun's Java1.1 JDK java.util.HashTable.java
* All "synchronized" keywords and some methods we do not need have been
* all been removed.
*/请问各位，现在的Hashtable是不是也并不是线程安全的了？为什么不再有synchronized修饰了？在网上也没查到什么，有没有什么相关资料可以给我推荐我阅读一下的，谢谢！
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李德胜1995 2017-07-31
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笑死。。。
u011165933 2017-07-31
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还请问下，那为什么现在没有synchronized关键字修饰了，如何保证线程安全的呢？[/quote]
你告诉我你在哪里看到的没有synchronized关键字啊？？[/quote]
比如put和get方法：
看的不是util包下的，没有问题了
public Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) throw new NullPointerException();
// Makes sure the key is not already in the hashtable.
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
Object old = e.value;
e.value = value;
return old;
}
}
// Rehash the table if the threshold is exceeded
if (count >= threshold) {
rehash();
return put(key, value);
}
// Creates the new entry.
e = new HashtableEntry();
e.hash = hash;
e.key = key;
e.value = value;
e.next = tab[index];
tab[index] = e;
count++;
return null;
}public Object get(Object key) {
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
return e.value;
return null;
}
/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param key the hashtable key
* @param value the value
* @return the previous value of the specified key in this hashtable,
* or <code>null</code> if it did not have one
* @exception NullPointerException if the key or value is
* <code>null</code>
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
soton_dolphin 2017-07-31
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还请问下，那为什么现在没有synchronized关键字修饰了，如何保证线程安全的呢？[/quote]
你告诉我你在哪里看到的没有synchronized关键字啊？？[/quote]
比如put和get方法：
public Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) throw new NullPointerException();
// Makes sure the key is not already in the hashtable.
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
Object old = e.value;
e.value = value;
return old;
}
}
// Rehash the table if the threshold is exceeded
if (count >= threshold) {
rehash();
return put(key, value);
}
// Creates the new entry.
e = new HashtableEntry();
e.hash = hash;
e.key = key;
e.value = value;
e.next = tab[index];
tab[index] = e;
count++;
return null;
}public Object get(Object key) {
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
return e.value;
return null;
}
/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param key the hashtable key
* @param value the value
* @return the previous value of the specified key in this hashtable,
* or <code>null</code> if it did not have one
* @exception NullPointerException if the key or value is
* <code>null</code>
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
u011165933 2017-07-31
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噢，我明白怎么回事了，我看的是com.sun.org.apache.xalan.internal.xsltc.runtime的Hashtable类，没注意是这个包下的，醉了
u011165933 2017-07-31
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我太菜了，大神给我解释下好呗
[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7[/quote]
1.7没有？？哪里看的？？？[/quote]
JDK源码中啊，没见到synchronized关键字，其中注释All "synchronized" keywords have been removed？
[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7[/quote]
1.7没有？？哪里看的？？？[/quote]
JDK源码中啊，没见到synchronized关键字，其中注释All "synchronized" keywords have been removed？u011165933 2017-07-31
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还请问下，那为什么现在没有synchronized关键字修饰了，如何保证线程安全的呢？[/quote]
你告诉我你在哪里看到的没有synchronized关键字啊？？[/quote]
比如put和get方法：
public Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) throw new NullPointerException();
// Makes sure the key is not already in the hashtable.
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
Object old = e.value;
e.value = value;
return old;
}
}
// Rehash the table if the threshold is exceeded
if (count >= threshold) {
rehash();
return put(key, value);
}
// Creates the new entry.
e = new HashtableEntry();
e.hash = hash;
e.key = key;
e.value = value;
e.next = tab[index];
tab[index] = e;
count++;
return null;
}public Object get(Object key) {
HashtableEntry e;
HashtableEntry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
return e.value;
return null;
}soton_dolphin 2017-07-31
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还请问下，那为什么现在没有synchronized关键字修饰了，如何保证线程安全的呢？[/quote]
你告诉我你在哪里看到的没有synchronized关键字啊？？
李德胜1995 2017-07-31
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我太菜了，大神给我解释下好呗[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7[/quote]
1.7没有？？哪里看的？？？
[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7[/quote]
1.7没有？？哪里看的？？？u011165933 2017-07-31
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我太菜了，大神给我解释下好呗[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7
[/quote]
你1.6的源码看不到synchronized？？？[/quote]
打错了是1.7李德胜1995 2017-07-31
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我太菜了，大神给我解释下好呗[/quote]
你1.6的源码看不到synchronized？？？
[/quote]
你1.6的源码看不到synchronized？？？u011165933 2017-07-31
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我太菜了，大神给我解释下好呗
u011165933 2017-07-31
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还请问下，那为什么现在没有synchronized关键字修饰了，如何保证线程安全的呢？
李德胜1995 2017-07-31
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soton_dolphin 2017-07-31
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一直都是线程安全的，你去API上一查就看到了 http://docs.oracle.com/javase/8/docs/api/
As of the Java 2 platform v1.2, this class was retrofitted to implement the Map interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.
