没有问题，因为update会先用当前读查出要修改的记录，然后再进行修改操作，而当前读是会对这些数据行加排它锁的，所以可以保证并发安全。下面是我写的一个小例子，有兴趣的可以自己实验一下，c1虽然早早就完成了更新，但是必然要等到c1睡眠时间结束后提交事务释放了排它锁，c2才能成功更新，在这之前c2是阻塞等待c1释放排它锁的
public static void main(String[] args) throws ClassNotFoundException, SQLException {
Class.forName("com.mysql.cj.jdbc.Driver");
Connection c1 = null;//获取连接1
Connection c2 = null;//获取连接2
c1.setAutoCommit(false);
c2.setAutoCommit(false);
new Thread() {
public void run() {
System.out.println("c1开始更新");
try {
PreparedStatement ps = c1.prepareStatement("update test set num = num -1 where id = 1");
ps.executeUpdate();
ps.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("c1完成更新");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("c1睡眠结束");
try {
c1.commit();
c1.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
new Thread() {
public void run() {
System.out.println("进入线程2");
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
System.out.println("c2开始更新");
try {
PreparedStatement ps = c2.prepareStatement("update test set num = num -1 where id = 1");
ps.executeUpdate();
ps.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("c2完成更新");
try {
c2.commit();
c2.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
}