Java supports asynchronous programming including the use of CompletableFuture, responsive streams (such as ProjectReactor), and virtual threads in Java19. 1.CompletableFuture improves code readability and maintenance through chain calls, and supports task orchestration and exception handling; 2. ProjectReactor provides Mono and Flux types to implement responsive programming, with backpressure mechanism and rich operators; 3. Virtual threads reduce concurrency costs, are suitable for I/O-intensive tasks, and are lighter and easier to expand than traditional platform threads. Each method has applicable scenarios, and appropriate tools should be selected according to your needs and mixed models should be avoided to maintain simplicity

JavaNIO is a new IOAPI introduced by Java 1.4. 1) is aimed at buffers and channels, 2) contains Buffer, Channel and Selector core components, 3) supports non-blocking mode, and 4) handles concurrent connections more efficiently than traditional IO. Its advantages are reflected in: 1) Non-blocking IO reduces thread overhead, 2) Buffer improves data transmission efficiency, 3) Selector realizes multiplexing, and 4) Memory mapping speeds up file reading and writing. Note when using: 1) The flip/clear operation of the Buffer is easy to be confused, 2) Incomplete data needs to be processed manually without blocking, 3) Selector registration must be canceled in time, 4) NIO is not suitable for all scenarios.

In Java, enums are suitable for representing fixed constant sets. Best practices include: 1. Use enum to represent fixed state or options to improve type safety and readability; 2. Add properties and methods to enums to enhance flexibility, such as defining fields, constructors, helper methods, etc.; 3. Use EnumMap and EnumSet to improve performance and type safety because they are more efficient based on arrays; 4. Avoid abuse of enums, such as dynamic values, frequent changes or complex logic scenarios, which should be replaced by other methods. Correct use of enum can improve code quality and reduce errors, but you need to pay attention to its applicable boundaries.

Anonymous internal classes are used in Java to create subclasses or implement interfaces on the fly, and are often used to override methods to achieve specific purposes, such as event handling in GUI applications. Its syntax form is a new interface or class that directly defines the class body, and requires that the accessed local variables must be final or equivalent immutable. Although they are convenient, they should not be overused. Especially when the logic is complex, they can be replaced by Java8's Lambda expressions.

Singleton design pattern in Java ensures that a class has only one instance and provides a global access point through private constructors and static methods, which is suitable for controlling access to shared resources. Implementation methods include: 1. Lazy loading, that is, the instance is created only when the first request is requested, which is suitable for situations where resource consumption is high and not necessarily required; 2. Thread-safe processing, ensuring that only one instance is created in a multi-threaded environment through synchronization methods or double check locking, and reducing performance impact; 3. Hungry loading, which directly initializes the instance during class loading, is suitable for lightweight objects or scenarios that can be initialized in advance; 4. Enumeration implementation, using Java enumeration to naturally support serialization, thread safety and prevent reflective attacks, is a recommended concise and reliable method. Different implementation methods can be selected according to specific needs

String is immutable, StringBuilder is mutable and non-thread-safe, StringBuffer is mutable and thread-safe. 1. Once the content of String is created cannot be modified, it is suitable for a small amount of splicing; 2. StringBuilder is suitable for frequent splicing of single threads, and has high performance; 3. StringBuffer is suitable for multi-threaded shared scenarios, but has a slightly lower performance; 4. Reasonably set the initial capacity and avoid using String splicing in loops can improve performance.

The COALESCE function is used to return the first non-null value in the parameter list and is suitable for processing NULL data. 1. The basic usage is to replace the NULL value, such as replacing the empty field with the default contact method; 2. It can be used to set the default value in aggregate query to ensure that 0 is returned instead of NULL when there is no data; 3. It can be used in conjunction with other functions such as NULLIF and IFNULL to enhance data cleaning and logical judgment capabilities.

Annotation processor is an extended mechanism in the Java compilation stage, used to scan and process annotations in the source code, and can generate new code or preprocess it. Its core functions include: 1. When defining annotations, it needs to specify the retention policy and target element type; 2. Implement the AbstractProcessor class and rewrite key methods such as getSupportedAnnotationTypes, getSupportedSourceVersion and process; 3. Register the processor to declare a fully qualified name through a configuration file in the META-INF/services directory. Annotation processors are widely used in frameworks such as Dagger, ButterKnife and Roo