1. AtomicInteger不是final类型，如何保证线程安全？

先看一下AtomicInteger类局部源码，关注两个字段：U以及value

public class AtomicInteger extends Number implements java.io.Serializable {
   
    private static final long serialVersionUID = 6214790243416807050L;

    /*
     * This class intended to be implemented using VarHandles, but there
     * are unresolved cyclic startup dependencies.
     */
    private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
    private static final long VALUE = U.objectFieldOffset(AtomicInteger.class, "value");

    private volatile int value;

    /**
     * Creates a new AtomicInteger with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicInteger(int initialValue) {
   
        value = initialValue;
    }

    /**
     * Creates a new AtomicInteger with initial value {@code 0}.
     */