集合的线程安全setmap

set HashSet

package com.dongguo.threadsafe;

import java.util.HashSet;
import java.util.Set;
import java.util.UUID;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:28
 * @description: Set集合线程不安全演示
 */
public class ThreadDemo2 {
    public static void main(String[] args) {
        Set set = new HashSet();

        for (int i = 0; i {
                set.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(set);
            }, "t" + i).start();
        }
    }
}

运行结果

HashSet的add方法

public boolean add(E var1) {
    return this.map.put(var1, PRESENT) == null;
}

其实调用的是HashMap的put()方法

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

HashSet的底层使用的就是HashMap，具体可转到HashMap的put()方法

Collections.synchronizedSet()

package com.dongguo.threadsafe;


import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:28
 * @description: Set集合线程安全演示
 */
public class ThreadDemo4 {
    public static void main(String[] args) {
        Set set = Collections.synchronizedSet(new HashSet());
        for (int i = 0; i {
                set.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(set);
            }, "t" + i).start();
        }
    }
}

运行结果

线程安全

        public boolean add(E e) {
            synchronized (mutex) {return c.add(e);}
        }

使用synchronized同步代码块来保证线程安全

CopyOnWriteArraySet

package com.dongguo.threadsafe;

import java.util.HashSet;
import java.util.Set;
import java.util.UUID;
import java.util.concurrent.CopyOnWriteArraySet;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:28
 * @description: Set集合线程安全演示
 */
public class ThreadDemo2 {
    public static void main(String[] args) {
//        Set set = new HashSet();
        Set set = new CopyOnWriteArraySet();
        for (int i = 0; i {
                set.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(set);
            }, "t" + i).start();
        }
    }
}

运行结果

线程安全

public boolean add(E e) {
    return al.addIfAbsent(e);
}

调用CopyOnWriteArrayList的addIfAbsent方法

public boolean addIfAbsent(E e) {
    Object[] snapshot = getArray();
    return indexOf(e, snapshot, 0, snapshot.length) >= 0 ? false :
        addIfAbsent(e, snapshot);
}

...
    private boolean addIfAbsent(E e, Object[] snapshot) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] current = getArray();
            int len = current.length;
            if (snapshot != current) {
                // Optimize for lost race to another addXXX operation
                int common = Math.min(snapshot.length, len);
                for (int i = 0; i = 0)
                        return false;
            }
            Object[] newElements = Arrays.copyOf(current, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }

使用ReentrantLock保证线程安全

map HashMap

package com.dongguo.threadsafe;

import java.util.HashMap;
import java.util.Map;
import java.util.UUID;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:43
 * @description: Map集合线程不安全演示
 */
public class ThreadDemo3 {
    public static void main(String[] args) {
        Map map = new HashMap();
        for (int i = 0; i {
                map.put(key,UUID.randomUUID().toString().substring(0, 8));
                System.out.println(map);
            }, "t" + i).start();
        }
    }
}

运行结果

HashMap并没有进行同步操作

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node[] tab; Node p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

在多线程环境下，使用HashMap进行put操作会引起死循环，导致CPU利用率接近100%，所以在并发情况下不能使用HashMap。

HashTable

package com.dongguo.threadsafe;


import java.util.Hashtable;
import java.util.Map;
import java.util.UUID;



/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:43
 * @description: Map集合线程安全演示
 */
public class ThreadDemo6 {
    public static void main(String[] args) {
        Map map = new Hashtable();
 
        for (int i = 0; i {
                map.put(key,UUID.randomUUID().toString().substring(0, 8));
                System.out.println(map);
            }, "t" + i).start();
        }
    }
}

运行结果

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 entry = (Entry)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;
}

synchronized同步方法

Collections.synchronizedMap()

package com.dongguo.threadsafe;


import java.util.*;
import java.util.concurrent.ConcurrentHashMap;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:43
 * @description: Map集合线程安全演示
 */
public class ThreadDemo5 {
    public static void main(String[] args) {
        Map map = Collections.synchronizedMap(new HashMap());
        for (int i = 0; i {
                map.put(key,UUID.randomUUID().toString().substring(0, 8));
                System.out.println(map);
            }, "t" + i).start();
        }
    }
}

运行结果

线程安全

public V put(K key, V value) {
    synchronized (mutex) {return m.put(key, value);}
}

使用synchronized同步代码块实现线程安全

ConcurrentHashMap

在并发编程中使用HashMap可能导致程序死循环，而使用线程安全的HashTable效率又非常低，基于这两个原因，ConcurrentHashMap出现了。

package com.dongguo.threadsafe;


import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;


/**
 * @author Dongguo
 * @date 2021/8/24 0024-10:43
 * @description: Map集合线程不安全演示
 */
public class ThreadDemo3 {
    public static void main(String[] args) {
//        Map map = new HashMap();
        Map map = new ConcurrentHashMap();
        for (int i = 0; i {
                map.put(key,UUID.randomUUID().toString().substring(0, 8));
                System.out.println(map);
            }, "t" + i).start();
        }
    }
}

运行结果

没有线程安全问题

ConcurrentHashMap的put()使用synchronized

ConcurrentHashMap通过CAS+Synchronized 来保证线程安全问题，

ConcurrentHashMap所使用的锁分段技术。首先将数据分成一段一段地存

储，然后给每一段数据配一把锁，当一个线程占用锁访问其中一个段数据的时候，其他段的数

据也能被其他线程访问。

synchronized只锁定当前链表或红黑树的首节点

public V put(K key, V value) {
    return putVal(key, value, false);
}

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node[] tab = table;;) {
        Node f; int n, i, fh;
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            if (casTabAt(tab, i, null,
                         new Node(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node e = f;; ++binCount) {
                            K ek;
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            Node pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    else if (f instanceof TreeBin) {
                        Node p;
                        binCount = 2;
                        if ((p = ((TreeBin)f).putTreeVal(hash, key,
                                                       value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            }
            if (binCount != 0) {
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}