四.集合安全性問題

ArrayList安全性問題

問題：在併發情況下，ArrayList可能會導致add的內容為空，丟擲java.util.ConcurrentModificationException(併發修改異常)。

解決方案：

• List list = new Vector();（Vector的add方法添加了synchronized鎖，該方法會影響效能）
• 點選檢視程式碼

  public class ListS {
      public static void main(String[] args) {
          List list = new Vector();
          for (int i = 0; i < 100; i++) {
              new Thread(()->{
                  list.add(UUID.randomUUID().toString().substring(0,3));
                  System.out.println(list);
              }).start();
          }
      }
  }
   
  

• List list = Collections.synchronizedList(new ArrayList());
• 點選檢視程式碼

  public class ListS {
      public static void main(String[] args) {
          List list = Collections.synchronizedList(new ArrayList());
          for (int i = 0; i < 100; i++) {
              new Thread(()->{
                  list.add(UUID.randomUUID().toString().substring(0,3));
                  System.out.println(list);
              }).start();
          }
      }
  }
   
  

• List list = new CopyOnWriteArrayList();
• 點選檢視程式碼

  public class ListS {
      public static void main(String[] args) {
          List list = new CopyOnWriteArrayList();
          for (int i = 0; i < 100; i++) {
              new Thread(()->{
                  list.add(UUID.randomUUID().toString().substring(0,3));
                  System.out.println(list);
              }).start();
          }
      }
  }
   
  

Set安全性問題

解決方案：

• Set set = new CopyOnWriteArraySet();
• 點選檢視程式碼

  public class SetS {
      public static void main(String[] args) {
          Set set = new CopyOnWriteArraySet();
          for (int i = 0; i < 150; i++) {
              new Thread(()->{
                  set.add(UUID.randomUUID().toString().substring(0,3));
                  System.out.println(set);
              }).start();
          }
      }
  }

• Set set = Collections.synchronizedSet(new HashSet());

Map安全性問題

解決方案：

• Map<String,String> map = Collections.synchronizedMap(new HashMap<>());
• 點選檢視程式碼

  public class MapS {
      public static void main(String[] args) {
          Map<String,String> map = Collections.synchronizedMap(new HashMap<>());
          for (int i = 0; i < 100; i++) {
              new Thread(()->{
                  map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,3));
                  System.out.println(map);
              }).start();
          }
      }
  }
  

• Map<String,String> map = new ConcurrentHashMap<>();
• 點選檢視程式碼

  public class MapS {
      public static void main(String[] args) {
          Map<String,String> map = new ConcurrentHashMap<>();
          for (int i = 0; i < 100; i++) {
              new Thread(()->{
                  map.put(Thread.currentThread().getName(), UUID.randomUUID().toString().substring(0,3));
                  System.out.println(map);
              }).start();
          }
      }
  }