说起来真特么惭愧：十年 IT 老兵，Java 菜鸟一枚。今天我才了解到 Java 还有 fail-fast 一说。不得不感慨啊，学习真的是没有止境。只要肯学，就会有巨多巨多别人眼中的“旧”知识涌现出来，并且在我这全是新的。
能怎么办呢？除了羞愧，就只能赶紧全身心地投入学习，把这些知识掌握。
为了镇楼，必须搬一段英文来解释一下 fail-fast。
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.
很长一段时间里，我都不明白为什么不能在 for each 循环里进行元素的 remove。今天我们就来借机来体验一把。
List<String> list = new ArrayList<>();
list.add("沉默王二");
list.add("沉默王三");
list.add("一个文章真特么有趣的程序员");
for (String str : list) {
if ("沉默王二".equals(str)) {
list.remove(str);
}
}
System.out.println(list);
这段代码看起来没有任何问题，但运行起来就糟糕了。
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)
at com.cmower.java_demo.str.Cmower3.main(Cmower3.java:14)
为毛呢？
03、分析问题的杀手锏
这时候就只能看源码了，ArrayList.java 的 909 行代码是这样的。
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
public Iterator<E> iterator() {
return new Itr();
}
Itr 实现了 Iterator 接口。
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
Itr() {}
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
}
也就是说 new Itr() 的时候 expectedModCount 被赋值为 modCount，而 modCount 是 List 的一个成员变量，表示集合被修改的次数。由于 list 此前执行了 3 次 add 方法，所以 modCount 的值为 3；expectedModCount 的值也为 3。
可当执行 list.remove(str) 后，modCount 的值变成了 4。
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
略微透露一下原因：break 后循环就不再遍历了，意味着 Iterator 的 next 方法不再执行了，也就意味着 checkForComodification 方法不再执行了，所以异常也就不会抛出了。
但是呢，当 List 中有重复元素要删除的时候，break 就不合适了。
2）for 循环
List<String> list = new ArrayList<>();
list.add("沉默王二");
list.add("沉默王三");
list.add("一个文章真特么有趣的程序员");
for (int i = 0, n = list.size(); i < n; i++) {
String str = list.get(i);
if ("沉默王二".equals(str)) {
list.remove(str);
}
}
for 循环虽然可以避开 fail-fast 保护机制，也就说 remove 元素后不再抛出异常；但是呢，这段程序在原则上是有问题的。为什么呢？
This exception may be thrown by methods that have detected concurrent
modification of an object when such modification is not permissible.
For example, it is not generally permissible for one thread to modify a Collectionwhile another thread is iterating over it. In general, the results of theiteration are undefined under these circumstances. Some Iteratorimplementations (including those of all the general purpose collection implementationsprovided by the JRE) may choose to throw this exception if this behavior isdetected. Iterators that do this are known as fail-fast iterators,as they fail quickly and cleanly, rather that risking arbitrary,non-deterministic behavior at an undetermined time in the future.
说起来真特么惭愧：十年 IT 老兵，Java 菜鸟一枚。今天我才了解到 Java 还有 fail-fast 一说。不得不感慨啊，学习真的是没有止境。只要肯学，就会有巨多巨多别人眼中的“旧”知识涌现出来，并且在我这全是新的。
能怎么办呢？除了羞愧，就只能赶紧全身心地投入学习，把这些知识掌握。
为了镇楼，必须搬一段英文来解释一下 fail-fast。
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.
很长一段时间里，我都不明白为什么不能在 for each 循环里进行元素的 remove。今天我们就来借机来体验一把。
List<String> list = new ArrayList<>();
list.add("沉默王二");
list.add("沉默王三");
list.add("一个文章真特么有趣的程序员");
for (String str : list) {
if ("沉默王二".equals(str)) {
list.remove(str);
}
}
System.out.println(list);
这段代码看起来没有任何问题，但运行起来就糟糕了。
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)
at com.cmower.java_demo.str.Cmower3.main(Cmower3.java:14)
为毛呢？
03、分析问题的杀手锏
这时候就只能看源码了，ArrayList.java 的 909 行代码是这样的。
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
public Iterator<E> iterator() {
return new Itr();
}
Itr 实现了 Iterator 接口。
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
Itr() {}
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
}
也就是说 new Itr() 的时候 expectedModCount 被赋值为 modCount，而 modCount 是 List 的一个成员变量，表示集合被修改的次数。由于 list 此前执行了 3 次 add 方法，所以 modCount 的值为 3；expectedModCount 的值也为 3。
可当执行 list.remove(str) 后，modCount 的值变成了 4。
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
略微透露一下原因：break 后循环就不再遍历了，意味着 Iterator 的 next 方法不再执行了，也就意味着 checkForComodification 方法不再执行了，所以异常也就不会抛出了。
但是呢，当 List 中有重复元素要删除的时候，break 就不合适了。
2）for 循环
List<String> list = new ArrayList<>();
list.add("沉默王二");
list.add("沉默王三");
list.add("一个文章真特么有趣的程序员");
for (int i = 0, n = list.size(); i < n; i++) {
String str = list.get(i);
if ("沉默王二".equals(str)) {
list.remove(str);
}
}
for 循环虽然可以避开 fail-fast 保护机制，也就说 remove 元素后不再抛出异常；但是呢，这段程序在原则上是有问题的。为什么呢？
This exception may be thrown by methods that have detected concurrent
modification of an object when such modification is not permissible.
For example, it is not generally permissible for one thread to modify a Collectionwhile another thread is iterating over it. In general, the results of theiteration are undefined under these circumstances. Some Iteratorimplementations (including those of all the general purpose collection implementationsprovided by the JRE) may choose to throw this exception if this behavior isdetected. Iterators that do this are known as fail-fast iterators,as they fail quickly and cleanly, rather that risking arbitrary,non-deterministic behavior at an undetermined time in the future.
