synchronized
使用系统重量级的锁

AtomicXXX
使用无锁-自旋锁，CAS-类似于乐观锁， 所以效率优于synchronized

LongAdder
分段锁，每段分别使用CAS原理递增，最后再合计。线程较多时，效率优于以上两种。

代码示范

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;
public class T02_AtomicVsSyncVsLongAdder {
    static long count2 = 0L;
    static AtomicLong count1 = new AtomicLong(0L);
    static LongAdder count3 = new LongAdder();

    public static void main(String[] args) throws Exception {
        Thread[] threads = new Thread[1000];

        for (int i = 0; i < threads.length; i++) {
            threads[i] =
                    new Thread(() -> {
                        for (int k = 0; k < 100000; k++) count1.incrementAndGet();
                    });
        }

        long start = System.currentTimeMillis();

        for (Thread t : threads) t.start();

        for (Thread t : threads) t.join();

        long end = System.currentTimeMillis();

        //TimeUnit.SECONDS.sleep(10);

        System.out.println("Atomic: " + count1.get() + " time " + (end - start));
        //-----------------------------------------------------------
        Object lock = new Object();

        for (int i = 0; i < threads.length; i++) {
            threads[i] =
                    new Thread(new Runnable() {
                        @Override
                        public void run() {

                            for (int k = 0; k < 100000; k++)
                                synchronized (lock) {
                                    count2++;
                                }
                        }
                    });
        }

        start = System.currentTimeMillis();

        for (Thread t : threads) t.start();

        for (Thread t : threads) t.join();

        end = System.currentTimeMillis();


        System.out.println("Sync: " + count2 + " time " + (end - start));


        //----------------------------------
        for (int i = 0; i < threads.length; i++) {
            threads[i] =
                    new Thread(() -> {
                        for (int k = 0; k < 100000; k++) count3.increment();
                    });
        }

        start = System.currentTimeMillis();

        for (Thread t : threads) t.start();

        for (Thread t : threads) t.join();

        end = System.currentTimeMillis();

        //TimeUnit.SECONDS.sleep(10);

        System.out.println("LongAdder: " + count1.longValue() + " time " + (end - start));

    }
    static void microSleep(int m) {
        try {
            TimeUnit.MICROSECONDS.sleep(m);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}