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What is Metaspace in Java 8?

Metaspace is a memory area introduced by Java 8 to replace PermGen to store class metadata. 1. It uses local memory and can be dynamically expanded by default; 2. It avoids memory overflow problems caused by PermGen fixed size; 3. The garbage collection mechanism is different, and it is triggered only when the metaspace is exhausted or useless; 4. It can be configured through parameters such as -XX:MaxMetaspaceSize; 5. Monitoring tools include jstat, VisualVM and JConsole; 6. In actual development, pay attention to dynamic class generation, class loader release and third-party library problems, and analyze dump files and update dependent libraries when necessary to optimize performance.

Jul 10, 2025 pm 01:24 PM

How to use the Java Cryptography Architecture (JCA)?

How to implement security features using JavaCryptographyArchitecture (JCA)? The answers are as follows: 1. Select the appropriate provider, such as the built-in SUN, SunJCE or third-party BouncyCastle (BC), and add Security.addProvider() according to the needs; 2. Use KeyPairGenerator to generate key pairs, such as RSA or EC algorithm; 3. Use Cipher class to perform encryption and decryption operations, pay attention to choosing the appropriate filling method; 4. Use MessageDigest to implement message digest, such as SHA-256 for data integrity verification; 5. Use KeySto

Jul 10, 2025 pm 01:21 PM

How to read and write files in Java I/O?

The most common way to read and write files in Java is to use the java.io package. The specific methods include: 1. Use FileReader and FileWriter to perform character-level reading and writing of text files, which is suitable for processing human-readable text content; 2. Use BufferedReader and BufferedWriter to provide a buffering mechanism to improve the efficiency of reading and writing text by line, which is suitable for log analysis and configuration file parsing; 3. Use FileInputStream and FileOutputStream to process binary files, which is suitable for copying pictures, network transmission and other scenarios. These classes provide flexible choices based on the data type and operation method. It is recommended to combine try-wit with

Jul 10, 2025 pm 01:20 PM

How to find the shortest path in a graph using Dijkstra's algorithm in Java?

The Dijkstra algorithm is used to solve the problem of single source shortest path in the graph, especially when the edge weight is positive. 1. Use an adjacency table to represent the graph structure, such as Map; 2. Initialize the distance array dist[], set the starting point to 0 and the rest to be infinity; 3. Use the priority queue to sort by the current distance and process the nodes in turn; 4. Take out the minimum distance node each time and update the distance of its neighbors; 5. Skip the nodes with the shortest path to improve efficiency; 6. Optional extensions include encapsulation graph construction process, recording predecessor nodes, optimizing data structures, etc.

Jul 10, 2025 pm 01:03 PM

What is GraphQL and how to use it with Java?

GraphQL is a query language and runtime framework for APIs developed and open sourced by Facebook in 2015 to solve the over-acquisition and under-acquisition problems in traditional RESTAPIs. It allows clients to request the required data accurately through a unified ingress. Java can be implemented through GraphQL-Java or SpringBootStarterforGraphQL; 1. Add dependencies, 2. Define Schema, 3. Write DataFetcher, 4. Create an execution engine, 5. Provide HTTP interface; design Schema should revolve around business entities to avoid excessive nesting; optimize data loading can use DataLoader to solve N 1 problems;

Jul 10, 2025 pm 12:56 PM

How to work with PDF files in Java using Apache PDFBox?

ApachePDFBox is a common tool for processing PDF files in Java, and supports creation, reading, merging and adding watermarks. 1. Create PDF: Use PDDocument and PDPageContentStream to add pages and write contents; 2. Read content: Extract text through PDFTextStripper, but the scanned file cannot be recognized; 3. Merge files: Use PDFMergerUtility to add multiple source files and merge output; 4. Add watermark: Create transparent layers after loading the document and draw watermark text or images on the specified page. Be sure to close the document object after the operation is completed to avoid memory leakage.

Jul 10, 2025 pm 12:45 PM

How to perform a breadth-first search (BFS) or depth-first search (DFS) on a graph in Java?

Implementing the BFS and DFS of graphs in Java mainly relies on adjacency tables to represent graphs, and use queues and recursion/stacks to control access order respectively. 1. The graph usually uses HashMap or ArrayList to store adjacency relationships; 2. DFS accesses each node recursively and marks accessed; 3. BFS uses a queue to access nodes by layer to ensure first-in-first-out; 4. The problems of null pointers, loops and non-connected graphs need to be handled.

Jul 10, 2025 pm 12:25 PM

Find duplicate elements in a Java array

To find duplicate elements in Java arrays, it can be achieved by loop counting, HashMap, or HashSet. 1. Use a nested loop to traverse the array and count, the time complexity is O(n2), which is suitable for small arrays; 2. Use HashMap to count the number of elements, the time complexity is O(n), which is suitable for large arrays; 3. Use HashSet to detect whether elements already exist, the time complexity is O(n), which is only judged whether there is duplication; 4. Pay attention to handling boundary situations such as empty arrays, and consider how to deal with the output form of multiple duplicate elements.

Jul 10, 2025 pm 12:17 PM

java array Repeating elements

Building RESTful APIs with Java Spring Boot

Using SpringBoot to build a RESTful API requires following resource naming specifications, HTTP method selection, Controller layer request processing, parameter binding method, unified response format and error handling mechanism. When designing an interface, you should focus on resource, such as /users represent user collection; select appropriate HTTP methods such as GET acquisition, POST creation, PUT update, DELETE deletion resources; use @RestController, @RequestMapping, @GetMapping, etc. to define interface paths and methods; bind through @PathVariable, @RequestParam, @RequestBody

Jul 10, 2025 pm 12:07 PM

How to profile a Java application for performance?

Java application performance analysis should first locate bottlenecks and then choose the appropriate method. 1. Use JDK's own tools such as jstat to view GC situation, jstack to troubleshoot thread problems, and jcmd for simple analysis; 2. Enable JFR to record runtime events, which is suitable for overall behavioral observation; 3. Use visual VM and other visual tools to intuitively view call stacks and hotspot methods; 4. Add monitoring buried points to the code to observe specific operations for a long time. Each method is suitable for different scenarios, and it is recommended to gradually and in-depth analysis from simple to traditional.

Jul 10, 2025 pm 12:06 PM

java Performance analysis

How to reverse a string in Java?

Inverting strings can be implemented in Java in a variety of ways. 1. The reverse() method of StringBuilder is the most recommended. The code is simple and efficient: newStringBuilder(original).reverse().toString(); 2. You can manually traverse the character array and exchange characters to achieve inversion, which helps you understand the underlying logic; 3. You can also use Java8Stream API to achieve functional style inversion, but the performance and readability are poor, which is only suitable for practice. The StringBuilder method is the first choice in actual development, and other methods can be selected and used according to specific needs.

Jul 10, 2025 am 11:58 AM

java String reverse

How to use Java Stream collect() with groupingBy?

The groupingBy collector of Stream in Java8 supports multiple grouping methods. ① Group by field: If you group by city, use Collectors.groupingBy(Person::getCity); ② Multi-level grouping: If you group by city first and then by age, use nested groupingBy; ③ Customize downstream operations: If you use Collectors.counting() to count the quantity, use Collectors.averagingInt() to calculate the average; ④ After grouping, merge data: If you splice the names into strings, use Collectors.mapping() to cooperate with Collectors.joini

Jul 10, 2025 am 11:53 AM

What are the best practices for writing concurrent Java code?

The following points should be followed by writing efficient and thread-safe concurrent Java code: 1. Use tool classes in the java.util.concurrent package, such as ConcurrentHashMap, CopyOnWriteArrayList and BlockingQueue, to improve performance and reliability; 2. Use thread pools (such as ExecutorService or ForkJoinPool) reasonably to manage thread resources, and set the appropriate number of threads according to the task type; 3. Avoid sharing mutable state, give priority to using immutable objects, and use atomic classes or locking mechanisms to ensure thread safety if necessary; 4. Pay attention to avoid deadlocks, live locks and resource hunger issues, and troubleshoot deadlocks can make it possible to

Jul 10, 2025 am 11:48 AM

java Concurrent programming

Exploring Concurrent Collections in Java util.concurrent

In a multi-threaded environment, using concurrent collections in the java.util.concurrent package can improve efficiency and security. 1.ConcurrentHashMap is suitable for high-concurrent read and write scenarios, and uses segmented locking or CAS mechanism to improve performance; 2.CopyOnWriteArrayList is suitable for List operations with more read and less read, such as event listener list; 3. BlockingQueue supports blocking operations and is often used in producer-consumer models; 4. Others such as ConcurrentSkipListMap, LinkedTransferQueue, etc. are also suitable for specific concurrent scenarios. When choosing, it should be based on read and write frequency, consistency requirements and other factors.

Jul 10, 2025 am 11:36 AM

java concurrent collection