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

Optional can clearly express intentions and reduce code noise for null judgments. 1. Optional.ofNullable is a common way to deal with null objects. For example, when taking values ??from maps, orElse can be used to provide default values, so that the logic is clearer and concise; 2. Use chain calls maps to achieve nested values ??to safely avoid NPE, and automatically terminate if any link is null and return the default value; 3. Filter can be used for conditional filtering, and subsequent operations will continue to be performed only if the conditions are met, otherwise it will jump directly to orElse, which is suitable for lightweight business judgment; 4. It is not recommended to overuse Optional, such as basic types or simple logic, which will increase complexity, and some scenarios will directly return to nu.

ThreadLocal is used in Java to create thread-private variables, each thread has an independent copy to avoid concurrency problems. It stores values ??through ThreadLocalMap inside the thread. Pay attention to timely cleaning when using it to prevent memory leakage. Common uses include user session management, database connections, transaction context, and log tracking. Best practices include: 1. Call remove() to clean up after use; 2. Avoid overuse; 3. InheritableThreadLocal is required for child thread inheritance; 4. Do not store large objects. The initial value can be set through initialValue() or withInitial(), and the initialization is delayed until the first get() call.

The core workaround for encountering java.io.NotSerializableException is to ensure that all classes that need to be serialized implement the Serializable interface and check the serialization support of nested objects. 1. Add implementsSerializable to the main class; 2. Ensure that the corresponding classes of custom fields in the class also implement Serializable; 3. Use transient to mark fields that do not need to be serialized; 4. Check the non-serialized types in collections or nested objects; 5. Check which class does not implement the interface; 6. Consider replacement design for classes that cannot be modified, such as saving key data or using serializable intermediate structures; 7. Consider modifying

There are three common methods to traverse Map in Java: 1. Use entrySet to obtain keys and values at the same time, which is suitable for most scenarios; 2. Use keySet or values to traverse keys or values respectively; 3. Use Java8's forEach to simplify the code structure. entrySet returns a Set set containing all key-value pairs, and each loop gets the Map.Entry object, suitable for frequent access to keys and values; if only keys or values are required, you can call keySet() or values() respectively, or you can get the value through map.get(key) when traversing the keys; Java 8 can use forEach((key,value)-&gt

Dynamic web crawling can be achieved through an analysis interface or a simulated browser. 1. Use browser developer tools to view XHR/Fetch requests in the Network, find the interface that returns JSON data, and use requests to get it; 2. If the page is rendered by the front-end framework and has no independent interface, you can start the browser with Selenium and wait for the elements to be loaded and extracted; 3. In the face of the anti-crawling mechanism, headers should be added, frequency control, proxy IP should be used, and verification codes or JS rendering detection should be carried out according to the situation. Mastering these methods can effectively deal with most dynamic web crawling scenarios.

In Java, Comparable is used to define default sorting rules internally, and Comparator is used to define multiple sorting logic externally. 1.Comparable is an interface implemented by the class itself. It defines the natural order by rewriting the compareTo() method. It is suitable for classes with fixed and most commonly used sorting methods, such as String or Integer. 2. Comparator is an externally defined functional interface, implemented through the compare() method, suitable for situations where multiple sorting methods are required for the same class, the class source code cannot be modified, or the sorting logic is often changed. The difference between the two is that Comparable can only define a sorting logic and needs to modify the class itself, while Compar

There are three common ways to parse JSON in Java: use Jackson, Gson, or org.json. 1. Jackson is suitable for most projects, with good performance and comprehensive functions, and supports conversion and annotation mapping between objects and JSON strings; 2. Gson is more suitable for Android projects or lightweight needs, and is simple to use but slightly inferior in handling complex structures and high-performance scenarios; 3.org.json is suitable for simple tasks or small scripts, and is not recommended for large projects because of its lack of flexibility and type safety. The choice should be decided based on actual needs.