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What is a Semaphore in Java concurrency?

AsemaphoreinJavaisasynchronizationtoolthatcontrolsaccesstosharedorlimitedresourcesthroughacquire()andrelease()operations.Itworksbymaintainingacountofpermits;threadsmustacquireapermitbeforeaccessingtheresource,andreleaseitafterward.1.Binarysemaphoresh

Jul 24, 2025 am 01:54 AM

java concurrency

how to get current date and time in java

There are three main ways to get the current time in Java: 1. Use java.util.Date to be suitable for simple scenarios. Use newDate() to get the current time and match SimpleDateFormat format; 2. It is recommended to use java.time.LocalDateTime introduced by Java8, and obtain the current time through LocalDateTime.now(), which supports flexible formatting and time zone processing; 3. Get time information with time zones recommended to use ZonedDateTime combined with ZoneId, such as ZonedDateTime.now(ZoneId.of("Asia/Shangh

Jul 24, 2025 am 01:53 AM

java date time

How to compile and run a Java program from the command line?

Yes, you can compile and run Java programs using the command line. First, make sure that the JDK is installed and verify the installation through javac-version and java-version; then create or locate the source code file ending in .java, such as HelloWorld.java; then use javacHelloWorld.java to compile and generate the .class file; finally run the program through javaHelloWorld (without the .class extension) to see the output result. It is necessary to pay attention to common problems such as the class name and file name, the main method is correct, and the processing of the package structure.

Jul 24, 2025 am 01:37 AM

Java Aspect-Oriented Programming (AOP) with AspectJ

To implement AOP programming in Java using AspectJ, you need to clearly define the sections, write point-cut expressions, master the recommended usage of Around, and choose the appropriate weaving method. When defining a section, create a class and add @Aspect annotation, specify the notification type with @Before, @After, etc., and define the interception range through the execution expression; it is recommended to start with a simple writing method, such as execution(com.example.service..*(..)) means to intercept all methods under the specified package, or combine conditions with annotations or logical operators; Around suggests that the most powerful, you need to call joinPoint.proceed(

Jul 24, 2025 am 01:35 AM

Building a Production-Ready RESTful API in Java

Use SpringBoot for rapid production-level settings, and use automatic configuration and embedded server to simplify development; 2. Verify inputs through BeanValidation, and use @ControllerAdvice to handle exceptions globally and return structured error messages; 3. Use JWT to combine SpringSecurity to achieve authentication and authorization, configure HTTPS and security headers to avoid hard-coded keys; 4. Integrate SLF4J, Micrometer, Prometheus and OpenTelemetry to implement logs, monitoring and link tracking, and expose health checks and indicator endpoints through actuator; 5. Use SpringDataJP

Jul 24, 2025 am 01:34 AM

java

Building Resilient Java Systems with Circuit Breakers

CircuitbreakersinJavaapplicationsmanagefailuresfromexternalservicesbytrippingwhenfailurethresholdsareexceeded,preventingcascadingoutages.1.Theyoperateinthreestates:Closed(normaloperation),Open(tripped,blockingrequests),andHalf-Open(testingservicereco

Jul 24, 2025 am 01:22 AM

Java AOT Compilation with GraalVM Native Image

GraalVMNativeImage is a technology that compiles Java applications into native machine code in advance, with faster startup speed and lower memory footprint. 1. It generates executable files through static analysis without the need for a JVM running environment; 2. The construction steps include installing GraalVM, installing native-image plug-in, preparing executable JAR and running native-image commands; 3. Pay attention to the characteristics of reflection, dynamic proxy, etc. that require manual configuration or tool support; 4. It is recommended to use a lightweight framework and control dependencies to improve construction efficiency and compatibility.

Jul 24, 2025 am 01:03 AM

how to round a double to 2 decimal places in java

In Java, there are three common methods for rounding double type values to two decimals: 1. Use Math.round() to simply round, and implement it by multiplying by 100, rounding and then dividing by 100. It is suitable for basic operations but cannot control the rounding mode; 2. Use DecimalFormat to format the output, suitable for display to users, format can be defined and localized, but the result is a string that is not suitable for subsequent calculations; 3. BigDecimal should be used for financial or high-precision requirements scenarios, providing complete rounding mode control to avoid floating point errors, but the syntax is more cumbersome. It is crucial to choose the right method according to actual needs. Misuse of errors can lead to accuracy problems or implicit errors.

Jul 24, 2025 am 12:54 AM

Building a Real-Time Chat Application with Java and WebSockets

The key steps in building a live chat application using Java and WebSockets include: 1) Defining WebSocket endpoints through @ServerEndpoint; 2) Using @OnOpen, @OnMessage and other annotations to handle lifecycle events; 3) Maintaining session collections and broadcasting messages in the ChatEndpoint class; 4) The front-end connects ws://localhost:8080/your-app/chat through JavaScript and sends and receives messages; 5) Using Maven to manage dependencies and deploy the application to Tomcat; 6) After starting Tomcat, test real-time communication in multiple browser windows. Complete implementation includes backend session tube

Jul 24, 2025 am 12:52 AM

how to check java version in cmd

To check the Java version, you can use the java-version command in CMD to view the JRE version used by the current system. If you need to view the JDK compiler version, use javac-version; if the command cannot be recognized, it may be that Java is not installed or environment variables are not configured, you need to go to the official website to download and install and add the Java bin directory to the system PATH; if multiple Java versions are installed, you can view the path order through the wherejava command, and switch the default version by modifying PATH or using the version management tool.

Jul 24, 2025 am 12:39 AM

cmd java version

Optimizing Java JDBC Database Interactions

Use connection pooling, PreparedStatement and batching, shutting down resources, adjusting transaction boundaries and isolation levels to optimize JDBC performance. 1. Use connection pools (such as HikariCP) to reduce the overhead of frequently creating connections; 2. Use PreparedStatement to prevent SQL injection and improve execution efficiency, and combine batch processing to improve batch operation throughput; 3. Use try-with-resources to ensure that resources are automatically closed to avoid memory leaks; 4. Adjust transaction boundaries, close auto-commit and commit uniformly, and select appropriate transaction isolation levels based on the business to reduce lock competition.

Jul 24, 2025 am 12:21 AM

How to get the current date and time in Java 8?

In Java 8, you recommend using the classes in the java.time package; 1. Get the full date and time to use LocalDateTime.now(); 2. Get the date only with LocalDate.now(); 3. Get the time only with LocalTime.now(); 4. Format output needs to be matched with DateTimeFormatter; 5. Specify the time zone and pass the ZoneId parameters, such as ZoneId.of("Asia/Shanghai"); these are more intuitive, thread-safe and easier to use than the old Date and Calendar.

Jul 23, 2025 am 04:06 AM

How to read Java bytecode?

To understand Javabytecode, you can use the JDK-provided Javap tool to disassemble the bytecode to view the bytecode; 1. Use javac to compile the class file and view the method instruction list through the javap-c command; 2. Understand the stack-based bytecode structure and the operating mechanisms of common instructions such as iconst, store, iload, iadd, etc.; 3. You can use graphical tools such as BytecodeViewer or IDE plug-in to assist in analyzing the class structure and field information; 4. Pay attention to the actual conversion form of Java syntax sugar in bytecode, such as switch string support, try-with-resources and lambda expressions. Mastering these key points is helpful

Jul 23, 2025 am 04:05 AM

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how to sort an array in java

A common way to sort arrays in Java is to use Arrays.sort(). For basic data type arrays, such as int[] or double[], call Arrays.sort() directly to achieve ascending sort; if you need to sort in descending order, you need to use a wrapper class (such as Integer) and pass it into the Collections.reverseOrder() comparator. String arrays are sorted in dictionary order by default, and case-insensitive sorting can be achieved through String.CASE_INSENSITIVE_ORDER. When sorting custom object arrays, you need to make the class implement a Comparable interface or provide a Comparator, for example, according to Pe

Jul 23, 2025 am 04:03 AM