How to solve the ConcurrentModificationException exception warning in java

Exception analysis

I believe that anyone who has written some Java code has encountered this exception, which is generally caused by the following code:

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

public class Test{
    public static void main(String[] args){
      List<String> list = new ArrayList<>();
      list.add("123");
      list.add("456");
      list.add("789");
      for(String obj : list){
          list.remove(obj);
      }
    }
}

The above code will eventually throw java.util.ConcurrentModificationException, so why? First, we decompile the above code and get the following results (you can ignore it if you know the foreach syntax sugar better):

public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 4: 0

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/util/ArrayList
       3: dup
       4: invokespecial #3                  // Method java/util/ArrayList."<init>":()V
       7: astore_1
       8: aload_1
       9: ldc           #4                  // String 123
      11: invokeinterface #5,  2            // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
      16: pop
      17: aload_1
      18: ldc           #6                  // String 456
      20: invokeinterface #5,  2            // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
      25: pop
      26: aload_1
      27: ldc           #7                  // String 789
      29: invokeinterface #5,  2            // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
      34: pop
      35: aload_1
      36: invokeinterface #8,  1            // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
      41: astore_2
      42: aload_2
      43: invokeinterface #9,  1            // InterfaceMethod java/util/Iterator.hasNext:()Z
      48: ifeq          72
      51: aload_2
      52: invokeinterface #10,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
      57: checkcast     #11                 // class java/lang/String
      60: astore_3
      61: aload_1
      62: aload_3
      63: invokeinterface #12,  2           // InterfaceMethod java/util/List.remove:(Ljava/lang/Object;)Z
      68: pop
      69: goto          42
      72: return
    LineNumberTable:
      line 6: 0
      line 7: 8
      line 8: 17
      line 9: 26
      line 10: 35
      line 11: 61
      line 12: 69
      line 13: 72
}

Translating the above code is equivalent to the following code:

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

public class Test{
    public static void main(String[] args){
      List<String> list = new ArrayList<>();
      list.add("123");
      list.add("456");
      list.add("789");
      Iterator<String> iterator = list.iterator();
      while (iterator.hasNext()){
          String obj = iterator.next();
          list.remove(obj);
      }
    }
}

Then we Looking at the iterator.hasNext() source code, you can find that the first line calls the checkForComodification method. Let’s check this method:

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

in modCount != expectedModCount When this condition is established, a ConcurrentModificationException exception will be thrown. So how is this condition established?

1. First, we check the source of modCount and we can find that the value of modCount is equal to the size of the current List. When calling List When using the .remove method, modCount will also be reduced by 1 accordingly;

2. Then we check the source of expectedModCount, and we can see that it is constructing# When ##Iterator (the internal implementation of ArrayList is used here), there is a variable assignment, and the value of modCount is assigned to expectedModCount;

3. Finally, when we execute the loop and call the

List.remove method, modCount changes but expectedModCount does not change. When the first loop ends, it is deleted. When a data is ready to call the iterator.hasNext() method for the second time in a loop, the checkForComodification() method will throw an exception, because at this time the List modCount has changed to 2, and expectedModCount is still 3, so ConcurrentModificationException exception will be thrown;

So how to solve this problem? When we look at the source code of

java.util.ArrayList.Itr (Iterator implementation in ArrayList), we can find that there is a remove method in this iterator that can delete the current iteration element, and it will Modify modCount and expectedModCount at the same time, so that no exception will be thrown when performing the checkForComodification check. The source code of the remove method is as follows :

public void remove() {
    if (lastRet < 0)
        throw new IllegalStateException();
    checkForComodification();

    try {
        ArrayList.this.remove(lastRet);
        cursor = lastRet;
        lastRet = -1;
        expectedModCount = modCount;
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}

Among them,

ArrayList.this.remove(lastRet);This line will change the value of modCount, and the subsequent modification will be synchronously expectedModCount## The value of # is equal to the value of modCount; Now modify the program we started with as follows and it will run normally:

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

public class Test{
    public static void main(String[] args){
      List<String> list = new ArrayList<>();
      list.add("123");
      list.add("456");
      list.add("789");
      Iterator<String> iterator = list.iterator();
      while (iterator.hasNext()) {
          System.out.println("移除：" + iterator.next());
          iterator.remove();
      }
    }
}

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