JUC多线程及高并发10---集合类不安全

Dear 丶 2023-06-19 09:37 30阅读 0赞

# 集合类不安全之并发修改异常 #

问题：我们知道ArrayList是线程不安全，请编码写一个不安全的案例并给出解决方案。

package ArrayList;
    
    import java.util.ArrayList;
    
    /*
    * 集合类不安全的问题
    * ArrayList
    * */
    public class ContainerNotSafeDemo {
        public static void main(String[] args) {
            //ArrayList是对数组的包装
            //第一问当我new一个 ArrayList,底层是什么？--------数组
            //第二问什么类型的数组？--------这里为整型 <泛型>（当你new的时候，Constructs an empty list with an initial capacity of ten.）
        new ArrayList<Integer>();
        }
    }

接着看elementData是什么？

public ArrayList() {
            this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
        }

是一个 Object数组

transient Object[] elementData;

列表为什么为空？

因为DEFAULTCAPACITY\_EMPTY\_ELEMENTDATA为空

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

当add添加1

这时size+1,而size初始值为0.

public boolean add(E e) {
            ensureCapacityInternal(size + 1);  // Increments modCount!!
            elementData[size++] = e;
            return true;
        }

如果超过10，需要扩容。

### 举例----单线程程比较安全 ###

一般不会出错

package ArrayList;
    
    import java.util.ArrayList;
    import java.util.List;
    
    /*
    * 集合类不安全的问题
    * ArrayList
    * */
    public class ContainerNotSafeDemo {
        public static void main(String[] args) {
            //ArrayList是对数组的包装
            //第一问当我new一个 ArrayList,底层是什么？--------数组
            //第二问什么类型的数组？--------这里为整型（当你new的时候，Constructs an empty list with an initial capacity of ten.）
    //    new ArrayList<Integer>().add(1);
    
            List<String> list=new ArrayList<>();
            list.add("a");
            list.add("b");
            list.add("c");
            for (String element : list){
                System.out.println(element);
            }
        }
    }

结果

a
    b
    c
    
    Process finished with exit code 0

### 多线程  ###

public class ContainerNotSafeDemo {
        public static void main(String[] args) {
            //ArrayList是对数组的包装
            //第一问当我new一个 ArrayList,底层是什么？--------数组
            //第二问什么类型的数组？--------这里为整型（当你new的时候，Constructs an empty list with an initial capacity of ten.）
    //    new ArrayList<Integer>().add(1);
    
            List<String> list=new ArrayList<>();
    
    
    
            //多线程
            for (int i = 1; i <= 3; i++) {
                new Thread(()->{
                    list.add(UUID.randomUUID().toString().substring(0,8));
                    System.out.println(list);
                },String.valueOf(i)).start();
            }
        }
    }

结果不正确，但是没报错，证明了线程不安全

[null, 244aa3b3]
    [null, 244aa3b3, 3a1a3d91]
    [null, 244aa3b3]
    
    Process finished with exit code 0

### 当  i<=30时 ###

结果

"C:\Program Files\Java\jdk1.8.0_144\bin\java.exe" "-javaagent:F:\idea2019\IntelliJ IDEA 2019.1.3\lib\idea_rt.jar=49644:F:\idea2019\IntelliJ IDEA 2019.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;F:\039_面试视频\笔记\out\production\笔记;F:\039_面试视频\笔记\lib\lombok-1.16.20.jar" ArrayList.ContainerNotSafeDemo
    [e9b0e33b, ec95100c]
    [e9b0e33b, ec95100c, 15ae19fc]
    Exception in thread "14" [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2]
    Exception in thread "28" [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177]
    Exception in thread "23" [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292]
    Exception in thread "17" Exception in thread "15" [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849]
    Exception in thread "3" [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919, a1f02e5a]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919, a1f02e5a, e445a60c]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919, a1f02e5a, e445a60c, eb5bfb7c]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919, a1f02e5a, e445a60c, eb5bfb7c, e5d9be52]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a, 599149d9, 3e15749c, e8351919, a1f02e5a, e445a60c, eb5bfb7c, e5d9be52, 9a1301d0]
    [e9b0e33b, ec95100c, 15ae19fc, 9b4b0406, 67a91cb2, 97b61c27, 4d7ae7fc, 9e3c5177, bc7d6d89, 5ab2c07c, 593711f3, 1fad7d27, 1acc8cd2, decac026, 4b899326, 23c0c292, 22d8ff11, 9c663849, c77fdb58, 37efc073, 31ed5bbb, 3363a98a]
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    java.util.ConcurrentModificationException
    	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    	at java.util.ArrayList$Itr.next(ArrayList.java:851)
    	at java.util.AbstractCollection.toString(AbstractCollection.java:461)
    	at java.lang.String.valueOf(String.java:2994)
    	at java.io.PrintStream.println(PrintStream.java:821)
    	at ArrayList.ContainerNotSafeDemo.lambda$main$0(ContainerNotSafeDemo.java:32)
    	at java.lang.Thread.run(Thread.java:748)
    
    Process finished with exit code 0

其中 java.util.ConcurrentModificationException 是ArrayList线程不安全在高并发多线程访问下面的异常。

Concurrent-----并发

Modification----修改

ConcurrentModificationException -----并发修改的异常

# 解决的例子 #

1.线程安全用 Vector<>

线程不安全，不考虑用性能的ArrayList

List<String> list= new Vector<>();

2.其中了解collection、collections区别，前者为接口，后者为类

List<String> list=Collections.synchronizedList(new ArrayList<>());

3.CopyOnWriteArrayList<>();

List<String> list= new CopyOnWriteArrayList<>();

**1.故障现象  
    Java.util.ConcurrentModificationException  
2.导致原因  
   并发争抢修改导致，参考花名册签名情况。  
   一个人正在写入，另一个人过来抢夺，导致数据不一致异常。并发修改异常。**

** 3.解决方案  
   3.1  new  Vector<>();----------(List<String> list=new Vector<>();)  
   3.2   Collections.synchronizedList(new ArrayList<>());  
   3.3   new CopyOnWriteArrayList<>();  
4.优化建议（同样的错误不犯第二次）**

***笔记 ***

写实复制

CopyOnWrite容器即写实复制的容器。往一个容器添加元素的时候，不直接往当前容器object【】添加，而是先将当前容器object【】进行copy，复制出一个新的容器object【】 newElements，然后新的容器object【】newElements里添加元素，添加完元素之后，再将原容器的引用指向新的容器setArray（newElements）；这样做的好处是可以对copyonWrite容器进行并发的读，而不需要加锁，因为当前容器不会添加任何元素。所以copyonWrite容器也是一种读写分离的思想，读和写不同的容器

public boolean add(E e) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                Object[] elements = getArray();
                int len = elements.length;
                Object[] newElements = Arrays.copyOf(elements, len + 1);
                newElements[len] = e;
                setArray(newElements);
                return true;
            } finally {
                lock.unlock();
            }
        }