Thread safe level

immutable

Immutable objects must be thread safe , And never need extra synchronization .

Java Most basic numerical classes in the class library are as follows Integer、String and BigInteger It's all immutable .

Unconditional thread safety

The constraints specified by the class specification are still valid when the object is accessed by multiple threads , No matter how the runtime environment is arranged , No threads

Need any additional synchronization .

Such as Random 、ConcurrentHashMap、Concurrent aggregate 、atomic

Conditional thread safety

Conditional thread safety classes can be thread safe for individual operations , But some sequences of operations may require external synchronization .

The most common example of conditional thread safety is traversal by Hashtable perhaps Vector Or the iterator returned

Non-thread safety ( Thread compatibility )

Thread compatible classes are not thread safe , But it can be used safely in a concurrent environment by using synchronization properly .

Such as ArrayList HashMap

Thread opposition

Thread opposites are those that are synchronized or not , Code that cannot be used concurrently in a multithreaded environment .

Ultimate Reality System.setOut()、System.runFinalizersOnExit()