java - 锁粒度

偏执的太偏执、 2022-09-12 09:52 225阅读 0赞

最近工作有个需求，需要加锁保证操作的原子性，但在一定程度上我想着可以根据业务类型对锁进行细化，于是简单的写了一个demo进行验证。  
先来看看synchronized的demo：

import java.util.concurrent.ConcurrentHashMap;
    
    public class MapLockDemo { 
        private static final ConcurrentHashMap<String, Object> lockMap = new ConcurrentHashMap<>();
    
        public static void main(String[] args) { 
            lockMap.put("key_1", new Object());
            new Thread(() -> { 
                try { 
                    syncLock1();
                } catch (InterruptedException e) { 
                    e.printStackTrace();
                }
            }).start();
            new Thread(() -> { 
                try { 
                    syncLock2();
                } catch (InterruptedException e) { 
                    e.printStackTrace();
                }
            }).start();
        }
    
        public static void syncLock1() throws InterruptedException { 
            Object key1 = lockMap.get("key_1");
            System.out.println("syncLock1获取中");
            synchronized (key1) { 
                System.out.println("syncLock1已获取");
                Thread.sleep(10000);
            }
            System.out.println("syncLock1已经释放");
        }
    
        public static void syncLock2() throws InterruptedException { 
            Object key1 = lockMap.get("key_1");
            System.out.println("syncLock2获取中");
            synchronized (key1) { 
                System.out.println("syncLock2已获取");
                Thread.sleep(10000);
            }
            System.out.println("syncLock2已经释放");
        }
    }

结果如下：  
![在这里插入图片描述][65bae24a3bc148b884243f635d9d0fa7.png]  
那么我们就可以维护一个锁池来进行细粒度的控制：

import java.util.HashMap;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    
    public class RepertoryService { 
        // key -> lockObject
        private final ConcurrentHashMap<String, Object> lockMap = new ConcurrentHashMap<>();
        //分段减的库存
        private final Map<String, Integer> quotaMap = new HashMap<>();
    
    
        public synchronized Object getLock(String key) { 
            Object lock = lockMap.get(key);
            if (lock == null) { 
                Object lockObject = new Object();
                lockMap.put(key, lockObject);
                return lockObject;
            } else { 
                return lock;
            }
            
        }
    
        public void addQuota(String key, int number) throws InterruptedException { 
            Object lock = getLock(key);
            synchronized (lock) { 
                Thread.sleep(100);
    // System.out.println(String.format("addQuota:{%s},key:{%s}", number, key));
                Integer quota = quotaMap.get(key);
                quota += number;
                quotaMap.put(key, quota);
                System.out.println(String.format("key -> {%s},now quota:{%s}", key, quota));
            }
        }
    
        public void addQuotaNoSync(String key, int number) throws InterruptedException { 
    
            Thread.sleep(100);
    // System.out.println(String.format("addQuota:{%s},key:{%s}", number, key));
            Integer quota = quotaMap.get(key);
            quota += number;
            quotaMap.put(key, quota);
            System.out.println(String.format("key -> {%s},now quota:{%s}", key, quota));
        }
    
        public void subQuota(String key, int number) throws InterruptedException { 
            Object lock = getLock(key);
            synchronized (lock) { 
                Thread.sleep(100);
    // System.out.println(String.format("addQuota:{%s},key:{%s}", number, key));
                Integer quota = quotaMap.get(key);
                quota -= number;
                quotaMap.put(key, quota);
                System.out.println(String.format("key -> {%s},now quota:{%s}", key, quota));
            }
        }
    
        public static void main(String[] args) { 
            RepertoryService service = new RepertoryService();
            ExecutorService pool = Executors.newFixedThreadPool(8);
            for (int i = 0; i < 100; i++) { 
                service.quotaMap.put(generateKey(i), 100);
            }
            for (int i = 0; i < 100; i++) { 
                int finnalI = i;
                pool.execute(() -> { 
                    try { 
                        service.addQuotaNoSync(generateKey(finnalI), 1);
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                });
            }
            pool.shutdown();
            System.out.println(service.quotaMap.toString());
        }
    
        public static String generateKey(int n) { 
            return "quota_" + n % 10;
        }
    }

对与不同key值获取不同的锁，这个key值可有业务模型进行生成。这里采用了一个简单的object作为synchronized的锁对象。其实也可以采用java自带的一些锁来作为lockMap的value部分。但在这里为了保证获取锁的线程安全，避免相同的key在同时获取锁的时候创建了多个不一样的锁，做了一个全局的获取锁的锁来保证线程安全。

其实再优化一下及对业务量有个预估，将这些锁预先创建出来，通过对key的hash值来hash到这些预先创建的锁上。那么就不用一个全局的获取锁的锁来保证锁的创建了。

# 但是如今java项目基本都是微服务架构，基本等同于废弃了synchronized和自带的Lock，以上方案在分布式下并不可用 #

[65bae24a3bc148b884243f635d9d0fa7.png]: /images/20220828/b8385f60758b4cc5b79149e0d0ad86a0.png