# 4、集合的线程安全问题
# 1、ArrayList线程不安全问题
# 1、演示
public static void main(String[] args) {
//复现线程不安全问题
List<Integer> noSale = new ArrayList<>();
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
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并发修改异常
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911)
at java.util.ArrayList$Itr.next(ArrayList.java:861)
at java.util.AbstractCollection.toString(AbstractCollection.java:461)
at java.lang.String.valueOf(String.java:2994)
at java.lang.StringBuilder.append(StringBuilder.java:131)
at com.hjc.demo4.ListTest.main(ListTest.java:18)
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# 2、三种解决方案
# 1、使用Vector线程安全的集合
List<Integer> noSale = new Vector<>();
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该解决方案自从jdk1.0出现，不常用
public static void main(String[] args) {
List<Integer> noSale = new Vector<>();
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
Vector集合中的所有方法都使用synchronized修饰
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# 2、使用Collections工具类获取线程安全的集合
Collections.synchronizedList(new ArrayList<>());
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该解决方案也比较古老，使用较少
public static void main(String[] args) {
List<Integer> noSale = Collections.synchronizedList(new ArrayList<>());
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
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# 3、使用juc中的CopyOnWriteArrayList解决
public static void main(String[] args) {
List<Integer> noSale = new CopyOnWriteArrayList<>();
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
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# 1、写时复制技术 -- CopyOnWriteArrayList
# 2、源码
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();
}
}
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# 2、HashSet线程不安全问题
# 1、演示
public static void main(String[] args) {
//复现线程不安全问题
Set<Integer> noSale = new HashSet<>();
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
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# 2、解决方案CopyOnWriteArraySet
使用JUC中的CopyOnWriteArraySet对象
public static void main(String[] args) {
//复现线程不安全问题
Set<Integer> noSale = new CopyOnWriteArraySet<>();
for (int i = 0; i < 30; i++) {
int finalI = i;
new Thread(() -> {
noSale.add(finalI);
System.out.println("noSale = " + noSale);
},"A").start();
}
}
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# 3、HashMap线程不安全问题
# 1、演示
public static void main(String[] args) {
//复现线程不安全问题
Map<String,String> noSale = new HashMap<>();
for (int i = 0; i < 30; i++) {
String finalI = String.valueOf(i);
new Thread(() -> {
String substring = UUID.randomUUID().toString().substring(0, 8);
System.out.println(substring);
noSale.put(finalI, substring);
System.out.println("noSale = " + noSale);
}).start();
}
}
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# 2、解决方案ConcurrentHashMap
使用JUC中的ConcurrentHashMap对象
public static void main(String[] args) {
Map<String,String> noSale = new ConcurrentHashMap<>();
for (int i = 0; i < 30; i++) {
String finalI = String.valueOf(i);
new Thread(() -> {
String substring = UUID.randomUUID().toString().substring(0, 8);
System.out.println(substring);
noSale.put(finalI, substring);
System.out.println("noSale = " + noSale);
}).start();
}
}
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