集合类与多线程

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非线程安全的java.util.ArrayList类

Main.java

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import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        new WriterThread(list).start();
        new ReaderThread(list).start();
    }
}

WriterThread.java

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import java.util.List;

public class WriterThread extends Thread {
    private final List<Integer> list;

    public WriterThread(List<Integer> list) {
        super("WriterThread");
        this.list = list;
    }
    
    public void run() {
        for (int i = 0; ; i++) {
            list.add(i);
            list.remove(0);
        }
    }
}

ReaderThread.java

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import java.util.List;

public class ReaderThread extends Thread {
    private final List<Integer> list;

    public ReaderThread(List<Integer> list) {
        super("ReaderThread");
        this.list = list;
    }

    public void run() {
        while (true) {
            for (int n : list) {
                System.out.println(n);
            }
        }
    }
}

result

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Exception in thread "ReaderThread" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
	at java.util.ArrayList$Itr.next(ArrayList.java:851)
	at ReaderThread.run(ReaderThread.java:13)

利用Collections.synchronizedList方法所进行的同步

Main.java

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import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        final List<Integer> list = Collections.synchronizedList(new ArrayList<>());
        new WriterThread(list).start();
        new ReaderThread(list).start();
    }
}

ReaderThread.java

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import java.util.List;

public class ReaderThread extends Thread {
    private final List<Integer> list;

    public ReaderThread(List<Integer> list) {
        super("ReaderThread");
        this.list = list;
    }

    public void run() {
        while (true) {
            synchronized(list) {
                for (int n : list) {
                    System.out.println(n);
                }
            }
        }
    }
}

使用copy-on-write的java.util.concurrent.CopyOnWriteArrayList类

Main.java

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import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class Main {

    public static void main(String[] args) {
        final List<Integer> list = new CopyOnWriteArrayList<>();
        new WriterThread(list).start();
        new ReaderThread(list).start();
    }
}

比较

使用copy-on-write时，每次执行“写”操作都会执行复制。因此，程序频繁执行“写”操作时，如果使用CopyOnWriteArrayList，会比较花费时间。但如果写操作比较少，而读操作频率非常高时，使用CopyOnWriteArrayList是非常棒的。