java virtual machine--thread safety and lock optimization

Thread safety

Thread safety in java language

　1 Immutable. Immutable objects must be thread-safe

　2 For a class to be absolutely thread-safe, no matter what the runtime environment is, the caller does not need additional synchronization measures. It usually requires a large or even unrealistic price to pay. In the Java API Classes marked themselves as thread-safe, most of them are not absolutely thread-safe

　3 Relative thread-safety

　4 Thread-compatible The object itself is not thread-safe, but it can be ensured by correctly using synchronization means on the calling end. It can be used safely in a concurrent environment

　5 Thread opposition

Thread-safe implementation method

　1 Murual Exclusion & Synchronization

　　Common concurrency correctness guarantee mobile phone, synchronization refers to when multiple threads concurrently access shared data. The storage certificate shared data is only used by one thread at the same time. The most basic mutual exclusion method is the synchronized keyword.

　　You can also use the lock in the java.util.concurrent package to achieve synchronization ReentrantLock,

　　 JDK1.6 has optimized synchronized, and the efficiency is basically the same as that of ReentrantLock.

　2 Non-blocking synchronization

　Optimistic concurrency strategy based on conflict detection

　CAS operation Compare-and-Swap

　3 No synchronization solution

Lock optimization

1. Spin lock and adaptive spin

　2. Lock elimination

　3. Lock coarsening

　4. Level lock

　5. Bias lock