synchronized, a common lock strategy

Synchronized: 
  1. Synchronized is an optimistic lock, but also a pessimistic lock (see the level of lock competition gradually escalating) 
  2. Synchronized is not a read-write lock, but an ordinary mutual exclusion lock 
  3. Synchronized is both a lightweight lock and a heavyweight Lock (see the level of lock competition gradually escalating) 
  4. Synchronized is a spin lock at the beginning (spin lock is also a lightweight lock), and then gradually upgrades 
  5. Unfair lock 
  6. Reentrant lock

The principle of synchronized: 
   1. The characteristics of synchronized 
      Atomicity 
      Memory visibility Orderedness 
      (instruction reordering) 
      Unfair locks 
      Reentrant locks 
      cannot be interrupted (not that the thread cannot be interrupted, but that when the lock cannot be obtained, this The blocking process cannot be interrupted) 
   2. The use of synchronized 
      Modifies code blocks 
      Modifies common methods 
      Modifies static methods 
   3. Synchronized lock mechanism 
      Lock upgrade, lock elimination, lock coarsening 
   4. Synchronized implementation 
      obtains monitor Every object has a monitor associated monitor => monitor lock

Synchronized lock upgrade: no lock -> biased lock -> spin lock (lightweight lock) -> heavyweight lock

Biased lock: It is not a real lock. It means that there is a certain bias towards a certain thread. The biased lock is a sign. In the object header, the thread to which the current lock is biased is stored. When a thread accesses, it will first judge whether this sign is equal to the current one. If the thread is equal to the current thread, then the lock will be obtained directly. If it is empty, the object header of the current lock will be set to itself. If it is not equal to itself, the lock will be upgraded.

Common locking strategies:

 Optimistic lock and pessimistic lock 
   Optimism and pessimism are a kind of attitude of operating data 
   Optimism: think that this data is not changed, so no lock is added, before modification, judge whether other threads have modified the data, and modify it without modification , if it is modified, stop the operation on the data. 
   Pessimistic: think that this data must have threads to operate, so before the operation, it will be locked first. 
   Optimistic lock and pessimistic lock are not good or bad, and there are suitable scenarios. 

   Optimistic lock: suitable for reading and writing Less scenarios, more reads (no conflicts) 
   pessimistic locks: suitable for scenarios with more writes and fewer reads, more data is written (more conflicts) 

   synchronized is both optimistic locks and pessimistic locks 
   . Optimistic locks, as the frequency of conflicts increase, gradually become pessimistic locks

 The operation of the read-write lock
  on data is mainly reading and writing
   1> two threads, all read operations on data => thread safety
   2> two threads, one read, one write => thread is not safe
   as long as there is writing, the thread is Unsafe
   For this reason, read-write locks appear:
   read-write locks, locks on data operations are divided into read locks and write locks.
   For data, read operations, add read locks
                     to write operations, add write locks
   to read locks + read Locks are not mutually exclusive.
   Read lock + write lock are mutually exclusive.
   Write lock + read lock are mutually exclusive.
   Synchronized is an ordinary mutual exclusion lock, not a read-write lock

 Heavyweight locks and lightweight locks
   are relative
   heavyweight locks, which do more, and the performance is slower
   Lightweight locks, do less, and the performance is faster
   Most optimistic locks are lightweight locks
              Pessimistic locks are weight class lock is not absolute

   Most heavyweight locks need to call mutexes.
   Lightweight locks generally do not use
   synchronized. At first, they are lightweight locks. As conflicts increase, they will gradually be upgraded to heavyweight locks.

   Spin locks and pending wait locks

        Pseudo code of spin lock: non-real code
        while (lock grab (lock) == failure) {} If the lock grab fails, it will continue to loop until the lock grab is successful.
        Synchronized is also a spin lock at the beginning

    Fair lock and unfair lock
        synchronized is an unfair lock
        Fair lock: first come first
        served Unfair lock: Whoever grabs whose
        unfair lock is generally better than fair lock

   Reentrant lock and non-reentrant lock
         synchronized is a reentrant lock, that is, it will not lock itself to death
         Non-reentrant lock: it will lock itself