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How to use a Java profiler like JVisualVM or JProfiler?

The key to using JavaProfiler is to understand its functionality and follow the steps. 1. Start the tool and connect to the target application. JVisualVM can run directly and automatically recognize local processes. JProfiler needs to be installed and supports remote connections. 2. Analyze CPU and memory. JProfiler provides "CallTree" and "HotSpots". JVisualVM samples CPU through "Sampler", and both can view memory trends and object allocations. 3. Position thread problems. JVisualVM checks status changes through "Threads" tag. JProfiler can detect deadlocks and display resource waiting conditions. 4. In combination with the external environment, check the problem.

Jul 10, 2025 pm 01:52 PM

What are common Java security vulnerabilities and how to prevent them?

Java security issues mainly include unsafe deserialization, SQL injection, sensitive information leakage and improper permission control. 1. Unsafe deserialization may trigger arbitrary code execution, and deserialization of untrusted data, using whitelisting mechanisms, or alternative formats such as JSON should be avoided. 2. SQL injection can be prevented through parameterized queries, precompiled statements and ORM framework. 3. Sensitive information leakage needs to be protected through log desensitization, encryption configuration, exception handling and HTTPS. 4. Improper permission control may lead to overprivileged access. Authentication, RBAC, server authentication should be enforced and unpredictable resource identifiers should be used. Developers' good coding habits and security awareness are the key to ensuring the security of Java applications.

Jul 10, 2025 pm 01:51 PM

Vulnerability prevention java security

Securing Java Applications with Spring Security Framework

The most common way to protect Java applications is to use the SpringSecurity framework, which lies in user authentication, permission control and security configuration. 1. User authentication supports forms, HTTPBasic and OAuth2. It is recommended to load user information from the database with UserDetailsService, and enable CSRF protection and verification code mechanism to enhance security; 2. Permission control is implemented through roles and permissions. Access can be restricted using @PreAuthorize annotation, and URL level restrictions can be configured through HttpSecurity to keep the role level clear and avoid confusion; 3. Security configuration should enable security headers, configure session management, and enable log auditing, and at the same time adjust potential risks carefully.

Jul 10, 2025 pm 01:50 PM

How to implement the Strategy design pattern in Java?

TheStrategydesignpatterninJavaallowsdefiningafamilyofalgorithms,encapsulatingeachone,andmakingtheminterchangeabletochangebehavioratruntime.1.Defineastrategyinterfacethatdeclaresthemethod(s)allstrategiesmustimplement.2.Implementconcretestrategiesthatp

Jul 10, 2025 pm 01:36 PM

How to create a custom collector for Java Streams?

TocreateacustomcollectorinJavaStreams,useCollector.of()withsupplier,accumulator,combiner,andfinisher.1.Supplierinitializesthecontainer.2.Accumulatoraddselementstoit.3.Combinermergescontainers,crucialforparallelstreams.4.Finisherconvertstheresulttothe

Jul 10, 2025 pm 01:34 PM

Common causes of Java NullPointerException and solutions.

NullPointerException (NPE) in Java is a common runtime exception caused by operating null references. It can be prevented by the following methods: 1. Avoid using it before initializing the object, add null checks before calling the method; 2. Document the methods that may return null and prioritize whether it is null, and use Optional classes reasonably; 3. Avoid automatic unboxing and throw exceptions, use types such as wrapper class default values ??or OptionalInt; 4. Identify the support of null by the collection, and filter null values ??before processing. The occurrence of NPE can be effectively reduced through good coding habits.

Jul 10, 2025 pm 01:33 PM

How to create an immutable class in Java?

Creating an immutable class in Java follows several key steps. 1. Declare the class as final to prevent inheritance from corruption of immutability; 2. Set all fields to privatefinal to ensure that external cannot be modified and the state remains unchanged after the object is created; 3. The setter method is not provided, only getters are retained for reading attributes; 4. Initialize all fields in the constructor and copy mutable objects deeply to avoid shallow copying problems; 5. Getter returns a copy of the mutable object instead of the original reference to prevent external modification from affecting the internal state; 6. Correctly implement equals and hashCode methods to ensure the consistency of behavior during use of container classes.

Jul 10, 2025 pm 01:27 PM

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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