在线程内迭代

Question

我需要迭代一个列表，将列表中的每个元素添加到一个新列表中，并从原始列表中删除该元素。我有三个线程在同一个原始列表上运行相同的进程。每个线程需要添加3本书，然后删除这3本书，每一本书都要经过循环。

这是我的代码：

    public class BookBoxer {  
  static List<Book> library = new ArrayList<>();

  static Librarian librarian1 = new Librarian();
  static Thread thread1 = new Thread(librarian1, "librarian1");

  static Librarian librarian2 = new Librarian();
  static Thread thread2 = new Thread(librarian2, "librarian2");

  static Librarian librarian3 = new Librarian();
  static Thread thread3 = new Thread(librarian3, "librarian3");


  public static void main(String[] args) {
    buldLibrary();

    thread1.start();
    thread2.start();
    thread3.start();

    System.out.println("Librarian 1: total books: "+ librarian1.getTotalBooks()+ 
        " || total value: $" + librarian1.getTotalValue());
    System.out.println("librarian 1 book: " + librarian1.box.toString());
    System.out.println();

    System.out.println("Librarian 2: total books: "+ librarian2.getTotalBooks()+ 
        " || total value: $" + librarian2.getTotalValue());
    System.out.println("librarian 2 book: " + librarian2.box.toString());
    System.out.println();

    System.out.println("Librarian 3: total books: "+ librarian3.getTotalBooks()+ 
        " || total value: $" + librarian3.getTotalValue());
    System.out.println("librarian 3 book: " + librarian3.box.toString());
    System.out.println();

    System.out.println("This is the new library " + library);

    System.out.println("The library has books: " + libraryHasBooks());
  }

  public static void buldLibrary() {
    int i = 1;
    while(i < 101) {
      Book book = new Book("Book #");
      library.add(book);
      book.incrementPrice();
      System.out.println(book.toString());
      i++;
    }
  }

  public static boolean libraryHasBooks() {
    if(library.isEmpty()) {
      return false;
    }
    return true;
  }

static class Librarian implements Runnable {
     private static long sleepTime = ((long)(Math.random() * 1000));
     private double totalValue;
     private int totalBooks = 0;
     List<Book> box = new ArrayList<>();

    private void getBook(List<Book> aLibrary) { 
      Iterator<Book> libraryIterator = aLibrary.iterator();
        if(libraryIterator.hasNext()) {
        box.add(libraryIterator.next());
        }
    }  

    private void removeBook(List<Book> aLibrary) { 
      Iterator<Book> libraryIterator = aLibrary.iterator();
      if(libraryIterator.hasNext()) {
      libraryIterator.next();
      libraryIterator.remove();
      }
    }

    private double getTotalValue() {
      Iterator<Book> boxIterator = box.iterator();
        while(boxIterator.hasNext()) {
        totalValue += boxIterator.next().getPrice();
        }
        return totalValue;
      }

    private int getTotalBooks() {
      for(Book i : box) {
        totalBooks++;
      } 
      return totalBooks;
    }

    public void performAction() { 
     for(int x = 1; x <= 3; x++) {
       this.getBook(library);
       this.removeBook(library);
     }
   }

    @Override
    public void run() { 
      while(libraryHasBooks())
        {
          this.performAction();
          try 
          {
            Thread.sleep(sleepTime);
          } catch (InterruptedException e) { e.printStackTrace(); }
        }
      }

  }
}

这是我的输出：

    Librarian 1: total books: 3 || total value: $3.75
librarian 1 book: [Name: Book #1 Price: $1.0, Name: Book #2 Price: $1.25, Name: Book #3 Price: $1.5]

Librarian 2: total books: 3 || total value: $6.0
librarian 2 book: [Name: Book #4 Price: $1.75, Name: Book #5 Price: $2.0, Name: Book #6 Price: $2.25]

Librarian 3: total books: 3 || total value: $8.25
librarian 3 book: [Name: Book #7 Price: $2.5, Name: Book #8 Price: $2.75, Name: Book #9 Price: $3.0]

This is the new library [Name: Book #10 Price: $3.25, Name: Book #11 Price: $3.5, Name: Book #12 Price: $3.75, Name: Book #13 Price: $4.0, Name: Book #14 Price: $4.25, Name: Book #15 Price: $4.5, Name: Book #16 Price: $4.75, Name: Book #17 Price: $5.0, Name: Book #18 Price: $5.25, Name: Book #19 Price: $5.5,....................

The library has books: true

Exception in thread "librarian3" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
    at java.util.ArrayList$Itr.next(ArrayList.java:791)
    at com.github.salvadore_jefferson.libraryapp.BookBoxer$Librarian.getBook(BookBoxer.java:80)
    at com.github.salvadore_jefferson.libraryapp.BookBoxer$Librarian.performAction(BookBoxer.java:109)
    at com.github.salvadore_jefferson.libraryapp.BookBoxer$Librarian.run(BookBoxer.java:118)
    at java.lang.Thread.run(Thread.java:722)

谢谢大家！！

Answer 1

有一个并发队列实现应该做您想做的事情(您的代码看起来更像是一个队列，而不是一个列表)。

如果你必须使用一个列表(嗯，一个队列是一个列表，但也许你有特殊的要求)，你将不得不同步你的方法，这将使整个线程的事情变得毫无意义--但是如果这是一个练习，也许这就是他们想教你的.如果是这样的话，不要在列表上循环(迭代器会关闭)，并在与列表交互的代码的每一部分周围放置一个同步(列表)。

同时，一定要将读取和删除放在相同的同步语句中--在编写代码时，要考虑防御。任何一个洞，其中的另一个线程可能滑入和“攻击”，你应该考虑！即使是像x++这样的东西，如果是在两个线程中，并且没有得到适当的保护，也可能会被搞砸。

Answer 2

一般不允许一个线程修改一个集合，而另一个线程则在它上迭代。请参阅ConcurrentModificationException

Answer 3

1. 您可以通过同步列表来完成它。
2. 您可以等待一个公共共享对象。