在Java中，可以通過以下幾種方式實現多線程同步：

1. 使用synchronized關鍵字：

synchronized關鍵字可以用于修飾方法或者代碼塊，當一個線程訪問被synchronized修飾的方法或代碼塊時，其他線程將無法訪問該方法或代碼塊。這樣可以確保同一時間只有一個線程能夠訪問共享資源。

示例：

public class SynchronizedExample {
    public static void main(String[] args) {
        Counter counter = new Counter();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
            }
        });

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Counter: " + counter.getCount());
    }
}

class Counter {
    private int count;

    public synchronized void increase() {
        count++;
    }

    public synchronized int getCount() {
        return count;
    }
}

2. 使用Lock接口和ReentrantLock類：

Java提供了Lock接口和ReentrantLock類來實現更靈活的線程同步。與synchronized相比，Lock提供了更高的靈活性，例如可以嘗試獲取鎖、定時獲取鎖和可中斷獲取鎖等操作。

示例：

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockExample {
    public static void main(String[] args) {
        Counter counter = new Counter();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
            }
        });

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Counter: " + counter.getCount());
    }
}

class Counter {
    private int count;
    private Lock lock = new ReentrantLock();

    public void increase() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }

    public int getCount() {
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }
}

3. 使用java.util.concurrent包中的高級同步工具：

Java提供了許多高級的同步工具，如Semaphore、CountDownLatch、CyclicBarrier等。這些工具可以幫助您更容易地實現復雜的多線程同步需求。

示例（使用CountDownLatch）：

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {
    public static void main(String[] args) {
        Counter counter = new Counter();
        CountDownLatch latch = new CountDownLatch(2);

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
                latch.countDown();
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 1000; i++) {
                    counter.increase();
                }
                latch.countDown();
            }
        });

        t1.start();
        t2.start();

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Counter: " + counter.getCount());
    }
}

class Counter {
    private int count;

    public synchronized void increase() {
        count++;
    }

    public synchronized int getCount() {
        return count;
    }
}

以上就是Java中實現多線程同步的幾種方法。在實際應用中，可以根據具體需求選擇合適的同步方式。