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How to profile CPU and memory usage of a Java application?

To understand the CPU and memory usage of Java applications, you can use the following methods: 1. Use VisualVM to view real-time performance data, including heap memory, GC situation and thread analysis; 2. Use jstat and jmap command line tools to diagnose GC behavior and generate heap snapshots; 3. Add monitoring logic to the code to estimate memory changes. These methods are applicable to graphical interface debugging, server environment inspection and specific logical observations, and can be flexibly selected according to the actual scenario.

Jul 12, 2025 am 02:08 AM

Java Performance Analysis CPU memory analysis

Difference between primitive and reference types?

JavaScript's data types are divided into primitive types and reference types, and the core difference is the storage method and assignment behavior. Primitive types include string, number, boolean, null, undefined, symbol, and bigint, which are immutable and passed by value, such as leta=10;letb=a; modifying b does not affect a. Reference types such as objects, arrays, and functions are mutable and passed by reference. For example, letobj1={name:"Tom"}; letobj2=obj1; Modifying obj2.name will affect obj1.name. Typeof can be used to determine the type, but please note that n

Jul 12, 2025 am 02:08 AM

What is an exception in Java?

AnexceptioninJavaisaneventthatdisruptsthenormalflowofaprogram,oftencausedbyprogrammingerrorsorexternalissues.1)ExceptionscanresultfrommistakeslikeArrayIndexOutOfBoundsExceptionorNullPointerException.2)Theycanalsostemfromexternalproblemssuchasmissingf

Jul 12, 2025 am 02:07 AM

What is garbage collection in Java?

GarbageCollection(GC)inJavaisanautomaticmemorymanagementprocessthatidentifiesandremovesunusedobjectstofreeupmemory.1)GCworksbydeterminingobjectreachabilityfromGCrootssuchasactivethreads,staticfields,andlocalvariables.2)Unreachableobjectsaremarkedforc

Jul 12, 2025 am 02:04 AM

Understanding the Java volatile Keyword Usage

The volatile keyword in Java often feels a bit abstract, especially for those who are new to concurrent programming. In fact, its function is very clear: to ensure the visibility of variables between multiple threads. That is to say, when one thread modifies the variable value modified by volatile, other threads can see the change immediately. It is not a master key to solve all concurrency problems, but it is very useful in some scenarios. Let’s take a look at how to use it and where it is suitable for use. When do you need to use volatile? The most typical application scenario is the status flag, such as controlling whether the thread continues to run: privatevolatilebooleanrunning=true;

Jul 12, 2025 am 01:50 AM

java volatile

What is the strictfp keyword in Java?

ThestrictfpkeywordinJavaensuresconsistentfloating-pointresultsacrossplatformsbyenforcingIEEE754compliance.1.Itappliestoclasses,interfaces,andmethods,restrictingintermediatecalculationstostandardprecision.2.Withoutstrictfp,JVMsmayusehigher-precisionre

Jul 12, 2025 am 01:44 AM

wait() vs sleep() in Java multithreading

The main difference between sleep() and wait() is the purpose and lock handling. 1.sleep() is a static method of the Thread class, used to pause thread for a period of time without releasing locks; it is suitable for simulation delays and other scenarios. 2. wait() is an instance method of the Object class. It must be used in synchronized. It will release the lock and wait for notifications from other threads; it is suitable for thread collaboration such as the producer-consumer model. 3.sleep() does not rely on synchronous blocks and does not require notify to wake up, while wait() must be waken by notify or notifyAll. 4. Both need to catch InterruptedException, but wait() needs to prevent virtuality

Jul 12, 2025 am 01:43 AM

java multithreading

What are the different types of garbage collectors in Java (G1, ZGC, Shenandoah)?

There are three mainstream collectors in Java's garbage collection mechanism: G1, ZGC and Shenandoah. 1.G1 is suitable for 4GB to tens of GB of heap memory, taking into account both throughput and response time, and the pause is controllable but not as low as the latter two; 2. ZGC is aimed at super large heap (TB level) and extremely low latency (

Jul 12, 2025 am 01:13 AM

java Garbage collector

What is the `volatile` keyword in Java used for?

volatile is used in Java to ensure the visibility and order of variables in a multi-threaded environment, but does not guarantee atomicity. Its core functions include: 1. Ensure that variable modifications are immediately visible to other threads and avoid inconsistencies caused by local cache; 2. Prevent instruction reordering and act as a memory barrier to maintain consistency in operation sequence; 3. Applicable to simple scenarios such as status flags, such as signal notifications in inter-thread communication; 4. Not suitable for composite operations, such as self-increment operations, if synchronized or AtomicInteger is still required to ensure atomicity. Therefore, volatile is available when only assignment or reading flag bits are required, while operations involving dependencies on the current value require a stricter synchronization mechanism.

Jul 12, 2025 am 12:57 AM

How to check if two strings are anagrams in Java?

There are two common methods for determining whether two strings are asymptotics. 1. Use Arrays.sort(): convert two strings into a character array, and compare whether they are equal after sorting, and the time complexity is O(nlogn); 2. Use character counting method: compare by counting the number of occurrences of each character, the time complexity is O(n), which is more efficient but slightly complicated. Both methods need to first determine whether the string length is the same, and then decide whether to handle case, space or non-alphabetical characters according to the requirements.

Jul 12, 2025 am 12:10 AM

java Anagrams

How to generate a random number in Java?

There are four common ways to generate random numbers in Java. 1. Use Math.random() to quickly obtain floating point numbers from 0 to 1, which is suitable for simple scenarios but cannot control seeds; 2. Use the Random class to generate multiple types of random numbers and support setting seeds, which are suitable for scenarios that require repeated tests; 3. It is recommended to use ThreadLocalRandom in a multi-threaded environment, which has better performance and no need to create instances manually; 4. When security needs are involved, SecureRandom should be used to provide stronger randomness guarantees but slower speed.

Jul 12, 2025 am 12:07 AM

java random number

How to use Java Stream flatMap()?

The purpose of flatMap() is to convert each element in the stream into a new stream and merge it into a unified stream. Its core uses include: 1. Processing nested collections, such as flattening List into List; 2. Split and combining strings, such as splitting strings by spaces or commas and collecting all words or tags; 3. Used differently from maps, map is a one-to-one map, while flatMap is a one-to-many map, suitable for scenes where one element needs to generate multiple elements.

Jul 11, 2025 am 03:17 AM

flatMap

What is the 'synchronized' keyword in Java?

synchronized is used in Java to control access to shared resources in a multi-threaded environment. Its main function is to ensure that only one thread can execute synchronous methods or code blocks at the same time, prevent race conditions and maintain data consistency. Specifically, 1. Synchronized realizes synchronization through a mutex mechanism, that is, threads must obtain the object monitor (lock) before executing synchronous code. If the lock is held by other threads, it needs to wait until it is released; 2. It can be applied to a method or code block, where the synchronization method locks the entire method (instance method locks the current instance, and static method locks the class object), while the synchronization code block provides finer granular control, locking only key parts; 3. Use recommendations include priority use of synchronous code blocks to improve

Jul 11, 2025 am 03:11 AM

How to fix java.lang.OutOfMemoryError: Metaspace?

The root cause of the java.lang.OutOfMemoryError: The Metaspace error is that the Metaspace area of ??the JVM is insufficient memory, which is usually due to loading a large number of classes, such as microservice frameworks, dynamic proxying and other scenarios. 1. Metaspace memory limit can be adjusted through -XX:MaxMetaspaceSize and -XX:MetaspaceSize; 2. Check for class loading leakage to avoid high-frequency generation of new classes and troubleshoot ClassLoader usage problems; 3. If Compressedclassspace overflow, you can increase the pressure by -XX:CompressedClassSpaceSize.

Jul 11, 2025 am 03:06 AM