Fail-Fast
Mustard什么是fail-fast
百度百科上是这么写的: fail-fast是Java集合(Collection)中的一种错误机制. 我觉得不太全面.
下面来看一下维基百科上这怎么写的:
In systems design, a fail-fast system is one which immediately reports at its interface any condition that is likely to indicate a failure. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly flawed process. Such designs often check the system’s state at several points in an operation, so any failures can be detected early. The responsibility of a fail-fast module is detecting errors, then letting the next-highest level of the system handle them.
从上面这段话就能看出来, fail-fast是在系统设计当中的一种错误检测机制, 一旦检测到可能发生错误, 就立即抛出异常, 程序不再继续运行.
集合中的fail-fast
首先, 来复现这个错误:
List<String> nameList = new ArrayList<String>() {{
add("张三");
add("李四");
add("小王");
add("丑八怪");
}};
for (String name : nameList) {
if (name.equals("丑八怪")) {
nameList.remove(name);
}
}运行上面的代码就会抛出这样的异常:
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
at java.util.ArrayList$Itr.next(ArrayList.java:859)上面的代码是在for-each中要删除集合中的元素而抛出的异常. 同样的, 增加(add())元素也会抛出这个异常.
异常产生的原因
来看一下源码:
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// ...
}
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
private void fastRemove(int index) {
modCount++;
// ...
}从这段代码可以发现, 对ArrayList的add(), remove(), clear()方法, 只要涉及到改变集合中的元素的个数的方法都会导致modCount的改变, 但是没有对expectedModCount进行改变.
final void checkForComodification() {
if (expectedModCount != modCount)
throw new ConcurrentModificationException();
}当expectModCount和modCount不同的时候, 就会抛出一开始出现的异常.
避免产生异常
1. 使用普通for循环进行操作
List<String> nameList = new ArrayList<String>() {{
add("张三");
add("李四");
add("小王");
add("丑八怪");
}};
for (int i = 0; i < nameList.size(); i++) {
String name = nameList.get(i);
if (name.equals("丑八怪")) {
nameList.remove(name);
}
}这样虽然不会报错, 但是可能会产生漏删的情况出现.
2. 使用Iterator进行操作
List<String> nameList = new ArrayList<String>() {{
add("张三");
add("李四");
add("小王");
add("丑八怪");
}};
Iterator<String> iterator = nameList.iterator();
while (iterator.hasNext()) {
if (iterator.next().equals("丑八怪")) {
iterator.remove();
}
}3. 其实使用for-each也可以
List<String> nameList = new ArrayList<String>() {{
add("张三");
add("李四");
add("小王");
add("丑八怪");
}};
for (String name : nameList) {
if (name.equals("丑八怪")) {
nameList.remove(name);
break;
}
}当集合中只删除一个元素的时候, 在删除操作之后立即break, 使循环不进入下一次遍历就可以了.