Java Multithreaded Programming (6) Singleton Mode and Multithreading

　　1. Load immediately / "Hungry Man Mode"

　　2. Lazy Loading / "Lazy Mode"

　　3. Use static built-in classes to implement the singleton pattern

　　Fourth, the singleton pattern implementation of serialization and deserialization

　　5. Use static code block to implement singleton pattern

　　Six, use the enum enumeration data type to implement the singleton pattern

　　Seven, improve the use of enum enumeration to achieve singleton mode