正如您自己发现的那样，当有人处理数据时，尤其是CopyOnWriteArrayList在遍历列表时，无法进行完全安全的更改。因为：每当您处理数据时，没有上下文可以确保在其他人更改列表数据之前执行访问列表的完整语句块。
因此，您必须为执行整个数据访问块的所有访问操作（也用于读取！）提供任何上下文（如同步）。例如：
ArrayList<String> list = getList();
synchronized (list) {
int index = list.indexOf("test");
// if the whole block would not be synchronized,
// the index could be invalid after an external change
list.remove(index);
}
或者对于迭代器：
synchronized (list) {
for (String s : list) {
System.out.println(s);
}
}
但是现在这种类型的同步出现了一个大问题：它很慢并且不允许多读访问。
因此，为数据访问构建自己的上下文会很有用。我将使用 ReentrantReadWriteLock 来允许多个读取访问并提高性能。
我对这个话题很感兴趣，我会为 ArrayList 做一个这样的上下文，并在我完成后附在此处。
2012 年 10 月 20 日 | 18:30 - 编辑：我使用 ReentrantReadWriteLock 为安全的 ArrayList 创建了自己的访问上下文。
首先，我将插入整个 SecureArrayList 类（大多数第一个操作只是覆盖和保护），然后我插入我的 Tester 类并解释用法。
我只是用一个线程测试了访问，而不是同时测试了很多，但我很确定它有效！如果没有，请告诉我。
安全数组列表：
package mydatastore.collections.concurrent;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.ReadLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;
/**
* @date 19.10.2012
* @author Thomas Jahoda
*
* uses ReentrantReadWriteLock
*/
public class SecureArrayList<E> extends ArrayList<E> {
protected final ReentrantReadWriteLock rwLock;
protected final ReadLock readLock;
protected final WriteLock writeLock;
public SecureArrayList() {
super();
this.rwLock = new ReentrantReadWriteLock();
readLock = rwLock.readLock();
writeLock = rwLock.writeLock();
}
// write operations
@Override
public boolean add(E e) {
try {
writeLock.lock();
return super.add(e);
} finally {
writeLock.unlock();
}
}
@Override
public void add(int index, E element) {
try {
writeLock.lock();
super.add(index, element);
} finally {
writeLock.unlock();
}
}
@Override
public boolean addAll(Collection<? extends E> c) {
try {
writeLock.lock();
return super.addAll(c);
} finally {
writeLock.unlock();
}
}
@Override
public boolean addAll(int index, Collection<? extends E> c) {
try {
writeLock.lock();
return super.addAll(index, c);
} finally {
writeLock.unlock();
}
}
@Override
public boolean remove(Object o) {
try {
writeLock.lock();
return super.remove(o);
} finally {
writeLock.unlock();
}
}
@Override
public E remove(int index) {
try {
writeLock.lock();
return super.remove(index);
} finally {
writeLock.unlock();
}
}
@Override
public boolean removeAll(Collection<?> c) {
try {
writeLock.lock();
return super.removeAll(c);
} finally {
writeLock.unlock();
}
}
@Override
protected void removeRange(int fromIndex, int toIndex) {
try {
writeLock.lock();
super.removeRange(fromIndex, toIndex);
} finally {
writeLock.unlock();
}
}
@Override
public E set(int index, E element) {
try {
writeLock.lock();
return super.set(index, element);
} finally {
writeLock.unlock();
}
}
@Override
public void clear() {
try {
writeLock.lock();
super.clear();
} finally {
writeLock.unlock();
}
}
@Override
public boolean retainAll(Collection<?> c) {
try {
writeLock.lock();
return super.retainAll(c);
} finally {
writeLock.unlock();
}
}
@Override
public void ensureCapacity(int minCapacity) {
try {
writeLock.lock();
super.ensureCapacity(minCapacity);
} finally {
writeLock.unlock();
}
}
@Override
public void trimToSize() {
try {
writeLock.lock();
super.trimToSize();
} finally {
writeLock.unlock();
}
}
//// now the read operations
@Override
public E get(int index) {
try {
readLock.lock();
return super.get(index);
} finally {
readLock.unlock();
}
}
@Override
public boolean contains(Object o) {
try {
readLock.lock();
return super.contains(o);
} finally {
readLock.unlock();
}
}
@Override
public boolean containsAll(Collection<?> c) {
try {
readLock.lock();
return super.containsAll(c);
} finally {
readLock.unlock();
}
}
@Override
public Object clone() {
try {
readLock.lock();
return super.clone();
} finally {
readLock.unlock();
}
}
@Override
public boolean equals(Object o) {
try {
readLock.lock();
return super.equals(o);
} finally {
readLock.unlock();
}
}
@Override
public int hashCode() {
try {
readLock.lock();
return super.hashCode();
} finally {
readLock.unlock();
}
}
@Override
public int indexOf(Object o) {
try {
readLock.lock();
return super.indexOf(o);
} finally {
readLock.unlock();
}
}
@Override
public Object[] toArray() {
try {
readLock.lock();
return super.toArray();
} finally {
readLock.unlock();
}
}
@Override
public boolean isEmpty() { // not sure if have to override because the size is temporarly stored in every case...
// it could happen that the size is accessed when it just gets assigned a new value,
// and the thread is switched after assigning 16 bits or smth... i dunno
try {
readLock.lock();
return super.isEmpty();
} finally {
readLock.unlock();
}
}
@Override
public int size() {
try {
readLock.lock();
return super.size();
} finally {
readLock.unlock();
}
}
@Override
public int lastIndexOf(Object o) {
try {
readLock.lock();
return super.lastIndexOf(o);
} finally {
readLock.unlock();
}
}
@Override
public List<E> subList(int fromIndex, int toIndex) {
try {
readLock.lock();
return super.subList(fromIndex, toIndex);
} finally {
readLock.unlock();
}
}
@Override
public <T> T[] toArray(T[] a) {
try {
readLock.lock();
return super.toArray(a);
} finally {
readLock.unlock();
}
}
@Override
public String toString() {
try {
readLock.lock();
return super.toString();
} finally {
readLock.unlock();
}
}
////// iterators
@Override
public Iterator<E> iterator() {
return new SecureArrayListIterator();
}
@Override
public ListIterator<E> listIterator() {
return new SecureArrayListListIterator(0);
}
@Override
public ListIterator<E> listIterator(int index) {
return new SecureArrayListListIterator(index);
}
// deligated lock mechanisms
public void lockRead() {
readLock.lock();
}
public void unlockRead() {
readLock.unlock();
}
public void lockWrite() {
writeLock.lock();
}
public void unlockWrite() {
writeLock.unlock();
}
// getters
public ReadLock getReadLock() {
return readLock;
}
/**
* The writeLock also has access to reading, so when holding write, the
* thread can also obtain the readLock. But while holding the readLock and
* attempting to lock write, it will result in a deadlock.
*
* @return
*/
public WriteLock getWriteLock() {
return writeLock;
}
protected class SecureArrayListIterator implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
@Override
public boolean hasNext() {
return cursor != size();
}
@Override
public E next() {
// checkForComodification();
int i = cursor;
if (i >= SecureArrayList.super.size()) {
throw new NoSuchElementException();
}
cursor = i + 1;
lastRet = i;
return SecureArrayList.super.get(lastRet);
}
@Override
public void remove() {
if (!writeLock.isHeldByCurrentThread()) {
throw new IllegalMonitorStateException("when the iteration uses write operations,"
+ "the complete iteration loop must hold a monitor for the writeLock");
}
if (lastRet < 0) {
throw new IllegalStateException("No element iterated over");
}
try {
SecureArrayList.super.remove(lastRet);
cursor = lastRet;
lastRet = -1;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException(); // impossibru, except for bugged child classes
}
}
// protected final void checkForComodification() {
// if (modCount != expectedModCount) {
// throw new IllegalMonitorStateException("The complete iteration must hold the read or write lock!");
// }
// }
}
/**
* An optimized version of AbstractList.ListItr
*/
protected class SecureArrayListListIterator extends SecureArrayListIterator implements ListIterator<E> {
protected SecureArrayListListIterator(int index) {
super();
cursor = index;
}
@Override
public boolean hasPrevious() {
return cursor != 0;
}
@Override
public int nextIndex() {
return cursor;
}
@Override
public int previousIndex() {
return cursor - 1;
}
@Override
public E previous() {
// checkForComodification();
int i = cursor - 1;
if (i < 0) {
throw new NoSuchElementException("No element iterated over");
}
cursor = i;
lastRet = i;
return SecureArrayList.super.get(lastRet);
}
@Override
public void set(E e) {
if (!writeLock.isHeldByCurrentThread()) {
throw new IllegalMonitorStateException("when the iteration uses write operations,"
+ "the complete iteration loop must hold a monitor for the writeLock");
}
if (lastRet < 0) {
throw new IllegalStateException("No element iterated over");
}
// try {
SecureArrayList.super.set(lastRet, e);
// } catch (IndexOutOfBoundsException ex) {
// throw new ConcurrentModificationException(); // impossibru, except for bugged child classes
// EDIT: or any failed direct editing while iterating over the list
// }
}
@Override
public void add(E e) {
if (!writeLock.isHeldByCurrentThread()) {
throw new IllegalMonitorStateException("when the iteration uses write operations,"
+ "the complete iteration loop must hold a monitor for the writeLock");
}
// try {
int i = cursor;
SecureArrayList.super.add(i, e);
cursor = i + 1;
lastRet = -1;
// } catch (IndexOutOfBoundsException ex) {
// throw new ConcurrentModificationException(); // impossibru, except for bugged child classes
// // EDIT: or any failed direct editing while iterating over the list
// }
}
}
}
SecureArrayList_Test：
package mydatastore.collections.concurrent;
import java.util.Iterator;
import java.util.ListIterator;
/**
* @date 19.10.2012
* @author Thomas Jahoda
*/
public class SecureArrayList_Test {
private static SecureArrayList<String> statList = new SecureArrayList<>();
public static void main(String[] args) {
accessExamples();
// mechanismTest_1();
// mechanismTest_2();
}
private static void accessExamples() {
final SecureArrayList<String> list = getList();
//
try {
list.lockWrite();
//
list.add("banana");
list.add("test");
} finally {
list.unlockWrite();
}
////// independent single statement reading or writing access
String val = list.get(0);
//// ---
////// reading only block (just some senseless unoptimized 'whatever' example)
int lastIndex = -1;
try {
list.lockRead();
//
String search = "test";
if (list.contains(search)) {
lastIndex = list.lastIndexOf(search);
}
// !!! MIND !!!
// inserting writing operations here results in a DEADLOCK!!!
// ... which is just really, really awkward...
} finally {
list.unlockRead();
}
//// ---
////// writing block (can also contain reading operations!!)
try {
list.lockWrite();
//
int index = list.indexOf("test");
if (index != -1) {
String newVal = "banana";
list.add(index + 1, newVal);
}
} finally {
list.unlockWrite();
}
//// ---
////// iteration for reading only
System.out.println("First output: ");
try {
list.lockRead();
//
for (Iterator<String> it = list.iterator(); it.hasNext();) {
String string = it.next();
System.out.println(string);
// !!! MIND !!!
// inserting writing operations called directly on the list will result in a deadlock!
// inserting writing operations called on the iterator will result in an IllegalMonitorStateException!
}
} finally {
list.unlockRead();
}
System.out.println("------");
//// ---
////// iteration for writing and reading
try {
list.lockWrite();
//
boolean firstAdd = true;
for (ListIterator<String> it = list.listIterator(); it.hasNext();) {
int index = it.nextIndex();
String string = it.next();
switch (string) {
case "banana":
it.remove();
break;
case "test":
if (firstAdd) {
it.add("whatever");
firstAdd = false;
}
break;
}
if (index == 2) {
list.set(index - 1, "pretty senseless data and operations but just to show "
+ "what's possible");
}
// !!! MIND !!!
// Only I implemented the iterators to enable direct list editing,
// other implementations normally throw a ConcurrentModificationException
}
} finally {
list.unlockWrite();
}
//// ---
System.out.println("Complete last output: ");
try {
list.lockRead();
//
for (String string : list) {
System.out.println(string);
}
} finally {
list.unlockRead();
}
System.out.println("------");
////// getting the last element
String lastElement = null;
try {
list.lockRead();
int size = list.size();
lastElement = list.get(size - 1);
} finally {
list.unlockRead();
}
System.out.println("Last element: " + lastElement);
//// ---
}
private static void mechanismTest_1() { // fus, roh
SecureArrayList<String> list = getList();
try {
System.out.print("fus, ");
list.lockRead();
System.out.print("roh, ");
list.lockWrite();
System.out.println("dah!"); // never happens cos of deadlock
} finally {
// also never happens
System.out.println("dah?");
list.unlockRead();
list.unlockWrite();
}
}
private static void mechanismTest_2() { // fus, roh, dah!
SecureArrayList<String> list = getList();
try {
System.out.print("fus, ");
list.lockWrite();
System.out.print("roh, ");
list.lockRead();
System.out.println("dah!");
} finally {
list.unlockRead();
list.unlockWrite();
}
// successful execution
}
private static SecureArrayList<String> getList() {
return statList;
}
}
编辑：我添加了几个测试用例来演示线程中的功能。上面的课程完美运行，我现在在我的主项目（Liam）中使用它：
private static void threadedWriteLock(){
final ThreadSafeArrayList<String> list = getList();
Thread threadOne;
Thread threadTwo;
final long lStartMS = System.currentTimeMillis();
list.add("String 1");
list.add("String 2");
System.out.println("******* basic write lock test *******");
threadOne = new Thread(new Runnable(){
public void run(){
try {
list.lockWrite();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
} finally {
list.unlockWrite();
}
}
});
threadTwo = new Thread(new Runnable(){
public void run(){
//give threadOne time to lock (just in case)
try {
Thread.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Expect a wait....");
//if this "add" line is commented out, even the iterator read will be locked.
//So its not only locking on the add, but also the read which is correct.
list.add("String 3");
for (ListIterator<String> it = list.listIterator(); it.hasNext();) {
System.out.println("String at index " + it.nextIndex() + ": " + it.next());
}
System.out.println("ThreadTwo completed in " + (System.currentTimeMillis() - lStartMS) + "ms");
}
});
threadOne.start();
threadTwo.start();
}
private static void threadedReadLock(){
final ThreadSafeArrayList<String> list = getList();
Thread threadOne;
Thread threadTwo;
final long lStartMS = System.currentTimeMillis();
list.add("String 1");
list.add("String 2");
System.out.println("******* basic read lock test *******");
threadOne = new Thread(new Runnable(){
public void run(){
try {
list.lockRead();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
} finally {
list.unlockRead();
}
}
});
threadTwo = new Thread(new Runnable(){
public void run(){
//give threadOne time to lock (just in case)
try {
Thread.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Expect a wait if adding, but not reading....");
//if this "add" line is commented out, the read will continue without holding up the thread
list.add("String 3");
for (ListIterator<String> it = list.listIterator(); it.hasNext();) {
System.out.println("String at index " + it.nextIndex() + ": " + it.next());
}
System.out.println("ThreadTwo completed in " + (System.currentTimeMillis() - lStartMS) + "ms");
}
});
threadOne.start();
threadTwo.start();
}