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How to fix java.lang.OutOfMemoryError: Metaspace?

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
What is the var keyword (local-variable type inference) in Java 10?

What is the var keyword (local-variable type inference) in Java 10?

ThevarkeywordinJava10enableslocal-variabletypeinference,lettingthecompilerdeducethevariable'stypefromitsinitializer.1.Itreducesredundancyandenhancesreadabilitywhenthetypeisevident.2.Itcanonlybeusedforlocalvariablesinsidemethods,notforfields,parameter

Jul 11, 2025 am 03:05 AM
var keyword Java 10
Java interface vs abstract class

Java interface vs abstract class

The interface is suitable for defining behavioral norms, while the abstractclass is suitable for sharing code and state. 1. The interface is completely abstract, only contains method signatures and constants, and supports default and static methods; abstract classes can contain specific implementations, member variables and constructors. 2. A class can only inherit one abstract class, but can implement multiple interfaces, which is suitable for multiple inheritance behaviors. 3. The interface implements default extension through the default method, and abstract classes implement it through traditional methods and access other members. 4. The interface emphasizes "what can be done" and is used as a contract for module interaction; abstract classes emphasize "what is" and "how to do it", which is suitable as template sharing structure.

Jul 11, 2025 am 03:02 AM
ArrayList vs LinkedList in Java

ArrayList vs LinkedList in Java

ArrayList is suitable for frequent access to elements, while LinkedList is suitable for frequent insertion or deletion of intermediate elements. 1. In terms of internal structure, ArrayList is implemented based on dynamic arrays, with continuous memory and supports fast index access; LinkedList is implemented based on bidirectional linked lists, with low random access efficiency and traversal search. 2. When inserting and deleting, ArrayList needs to move subsequent elements, and the time complexity is O(n); LinkedList only modifies the pointer and can reach O(1) at known node locations. 3. In usage scenarios, you need to quickly access the ArrayList; frequently add and delete the LinkedList in the middle; select the memory-sensitive ArrayList; modify the link during iteration

Jul 11, 2025 am 02:57 AM
java
Java pass by value or pass by reference?

Java pass by value or pass by reference?

Java is value passing. For basic types, the copy of the actual value is passed, and the modification within the method does not affect the external variables; for reference types, the copy of the reference address is passed, and modifying the object content within the method will affect the external object, but the reassignment reference is invalid. For example, modifying the int parameter of the changeValue method does not affect external variables; modifyList modifying the content of the list through reference copy; repointing to the new list in the reassignList does not affect external original reference. In development, try to reassign references within methods to change external objects, but modify the state by calling the object method.

Jul 11, 2025 am 02:56 AM
java Pass by value
How to create a thread in Java?

How to create a thread in Java?

There are two main ways to create threads in Java: inherit the Thread class and implement the Runnable interface. 1. Inheriting the Thread class is a direct way. By defining the class that inherits Thread and overriding the run() method, instantiating and calling start() to start the thread. However, this method is limited by the Java single inheritance mechanism and cannot directly share task objects; 2. Implementing the Runnable interface is more flexible. By implementing the run() method and passing the object into the Thread constructor, it supports multi-thread sharing of the same task object, avoiding inheritance restrictions, and conforms to the idea of ??interface-oriented programming; in addition, anonymous internal classes or Lambda expressions can be used to simplify the code, which is suitable for simplicity.

Jul 11, 2025 am 02:51 AM
How to solve the 'coin change' problem using dynamic programming in Java?

How to solve the 'coin change' problem using dynamic programming in Java?

How to solve the problem of coin change? Use the dynamic programming method, the specific steps are as follows: 1. Create a dp array with a size of amount 1 and initialize it to the maximum value, set dp[0]=0; 2. Iterate through each coin and update the dp array, iterate each coin from its face value to amount, and take the minimum value dp[i]=min(dp[i],dp[i-coin] 1); 3. Finally, check whether dp[amount] is still greater than amount, if so, return -1, otherwise return dp[amount]. This method can effectively find out the minimum number of coins required to form the target amount or judge that it cannot be achieved.

Jul 11, 2025 am 02:48 AM
What is the `final` keyword for classes?

What is the `final` keyword for classes?

In Java, using the final keyword to modify the class means that the class cannot be inherited. Specific reasons include: 1. Forced to keep specific behavior unchanged; 2. Protect sensitive logic or security-related code from being modified; 3. Ensure thread safety or immutability (such as String class). The main applicable scenarios are: 1. Security-sensitive classes; 2. Immutable classes; 3. Tools or auxiliary classes. If you try to inherit the final class, a compilation error will be raised. For example, after defining the final class Animal, the Dog class attempt to inherit will cause the compilation to fail. In addition, the methods of the final class cannot be overwritten because there is no inheritance relationship.

Jul 11, 2025 am 02:47 AM
How to connect to and use Redis with Java (e.g., using Jedis or Lettuce)?

How to connect to and use Redis with Java (e.g., using Jedis or Lettuce)?

To connect and use Redis in Java, you can choose Jedis or Lettuce client. 1.Jedis is simple and lightweight, suitable for small projects. You need to add dependencies and connect and operate Redis using synchronous methods; 2. Lettuce is more modern and supports asynchronous operations. You need to add dependencies and create connections through RedisClient and use synchronous or asynchronous APIs; 3. General recommendations include using connection pools, handling exceptions, serializing complex objects, and monitoring memory usage. The two solutions have their own advantages, and the choice depends on the specific needs.

Jul 11, 2025 am 02:19 AM
java redis
How to parse JSON in Java?

How to parse JSON in Java?

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.

Jul 11, 2025 am 02:18 AM
java json
What is the Java Memory Model?

What is the Java Memory Model?

Java Memory Model (JMM) is a set of rules to ensure the consistency of concurrent execution of Java programs on different platforms. 1. It improves performance through the division of main memory and working memory, but may lead to variable visibility problems; 2. JMM defines 8 operations to control memory interactions, such as read, load, use, assign, store, write, lock, unlock, and requires pairs to appear to ensure synchronization; 3. The volatile keyword guarantees visibility and orderliness, but does not guarantee atomicity, and is suitable for use in combination with state flags and CAS; 4. The happens-before principle provides a basis for judging memory visibility, including program order, lock, and volatile variables

Jul 11, 2025 am 02:17 AM
What is the difference between map and flatMap in Java Streams?

What is the difference between map and flatMap in Java Streams?

In Java streams, maps are suitable for one-to-one conversions, while flatMap is used for one-to-many conversions or flattened nested structures. For example, use map to convert a string list to uppercase, and each element generates a result; and flatMap can expand nested lists, such as converting List to single-class, or dealing with Optional values. The key difference is that map converts each element into a new element, while flatMap converts each element into a stream and then merges it into a stream. Common misunderstandings include misuse of maps to cause nested streams or obfuscating return types. At this time, the compiler should use flatMap instead.

Jul 11, 2025 am 02:13 AM
What is the diamond problem in Java?

What is the diamond problem in Java?

Thediamondproblemoccurswhenaclassinheritsfromtwoparentclassesthatbothinheritfromthesamegrandparentclass,causingambiguityinmethodresolution.1.Javaavoidsthisbynotallowingmultipleinheritanceofclasses.2.However,Javaallowsimplementingmultipleinterfaceswit

Jul 11, 2025 am 01:51 AM
What is a JWT and how to use it in a Java application?

What is a JWT and how to use it in a Java application?

The use of JWT in Java applications involves generation, parsing and verification of tokens, and its core is implemented through dependency libraries such as auth0/java-jwt. 1. Add Maven dependencies to introduce the java-jwt library; 2. Use the HMAC256 algorithm and key to generate a token containing the topic and declaration; 3. Build a validator to parse and verify the token signature; 4. Extract the declaration from the payload for permission judgment. In actual applications, it is necessary to safely store keys, enable HTTPS transmission, set the token expiration time, and integrate it with SpringSecurity to ensure the security and flexibility of authentication and authorization.

Jul 11, 2025 am 01:45 AM
java jwt

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