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How to create a serverless application with AWS Lambda and Java?

The key to building serverless applications using AWSLambda and Java is to write standard Java classes and package them as JAR files to upload, while paying attention to cold-start optimization. 1. Prepare JDK8 or 11, Maven, AWSCLI and IDE, and introduce Lambda core dependencies; 2. Write classes that implement the RequestHandler interface or use POJO input and output to define the entry method; 3. Use Maven plug-in to package it into fatjar, and deploy it through the console or AWSCLI; 4. Use lambda-local or unit test local debugging, and combine CloudWatch and X-Ray to monitor logs and performance after it is launched, and ensure that the IAM permission configuration is correct.

Jul 15, 2025 am 03:17 AM

How to read a .properties file in Java?

Reading .properties files in Java mainly uses the Properties class to cooperate with FileInputStream or class loader. 1. Use FileInputStream to load local files: Create a Properties instance, and call the load() method to load the file contents through FileInputStream; 2. Read resources from classpath: Use getResourceAsStream() method, pay attention to the path writing method; 3. Handle Chinese garbled problems: Java9 can specify encoding loading, Java8 and below need to manually wrap InputStreamReader to specify encoding. Also need attention

Jul 15, 2025 am 03:16 AM

What is the Java Memory Model (JMM) in detail?

JavaMemoryModel(JMM)ensuresvisibilityandorderingofmemoryoperationsacrossthreads.1.JMMdefineshowthreadsinteractthroughmemory,focusingonvisibilityandpredictableorderingratherthanphysicallayout.2.Itguaranteesthatchangesmadebyonethreadarevisibletoothersu

Jul 15, 2025 am 03:16 AM

Can the main method be overridden?

The main method in Java cannot be rewritten because it is a static method and belongs to a class rather than an instance. 2. Subclasses can define their own main method, but this is not a rewrite, but a new method with the same name. 3. The main method can be overloaded, but only publicstaticvoidmain(String[]) will be recognized by the JVM as the program entry. 4. Each class can have an independent main method, and the entry point is selected by specifying the class name at runtime.

Jul 15, 2025 am 03:14 AM

How to format a date in Java with SimpleDateFormat?

Create and use SimpleDateFormat requires passing in format strings, such as newSimpleDateFormat("yyyy-MM-ddHH:mm:ss"); 2. Pay attention to case sensitivity and avoid misuse of mixed single-letter formats and YYYY and DD; 3. SimpleDateFormat is not thread-safe. In a multi-thread environment, you should create a new instance or use ThreadLocal every time; 4. When parsing a string using the parse method, you need to catch ParseException, and note that the result does not contain time zone information; 5. It is recommended to use DateTimeFormatter and Lo

Jul 15, 2025 am 03:12 AM

java date formatting

What is the transient keyword in Java?

ThetransientkeywordinJavaisusedtoexcludefieldsfromtheobject'sserializedstate;1.Itpreventsspecificfieldsfrombeingpartofthebytestreamduringserialization;2.Itisusefulforexcludingsensitivedata,cachedvalues,orunserializableresources;3.Transientfieldsarese

Jul 15, 2025 am 03:12 AM

java

How to convert a Map to a List in Java?

In Java, the method of converting a Map to a List depends on the type of the list you want. 1. Get the key list: Use the keySet() method to combine the ArrayList constructor to extract the key, such as ListkeyList=newArrayList(map.keySet()); 2. Get the value list: Extract the value through the values() method, such as ListvalueList=newArrayList(map.values()); 3. Get the key-value pair list: Use the entrySet() method to obtain the entry collection, such as ListentryList=newArrayList(map.entrySet())

Jul 15, 2025 am 03:11 AM

Difference between `==` and `.equals()` in Java.

In Java, the main difference between == and .equals() is the content of comparison: 1.== compares whether the object's reference points to the same memory address; 2..equals()'s default behavior is the same as ==, but is usually rewritten to compare the content of the object. For example, the String class overrides .equals() to compare character sequences, and when using ==, it will return false due to different references. For basic types such as int, you can only use == for value comparison. In actual use, operators should be selected according to requirements: == when you need to determine whether two objects are the same instance, and .equals() when you need to compare logical values, and pay attention to handling null values to avoid exceptions.

Jul 15, 2025 am 03:11 AM

How does a HashMap work internally in Java?

HashMap implements key-value pair storage through hash tables in Java, and its core lies in quickly positioning data locations. 1. First use the hashCode() method of the key to generate a hash value and convert it into an array index through bit operations; 2. Different objects may generate the same hash value, resulting in conflicts. At this time, the node is mounted in the form of a linked list. After JDK8, the linked list is too long (default length 8) and it will be converted to a red and black tree to improve efficiency; 3. When using a custom class as a key, the equals() and hashCode() methods must be rewritten; 4. HashMap dynamically expands capacity. When the number of elements exceeds the capacity and multiplies by the load factor (default 0.75), expand and rehash; 5. HashMap is not thread-safe, and Concu should be used in multithreaded

Jul 15, 2025 am 03:10 AM

java hashmap

What is a memory leak in Java and how to find it?

Java memory leak refers to the object no longer used but cannot be recycled by GC, resulting in invalid memory usage. Common types include long-lifetime objects holding short-lifetime objects, listeners not logged out, static collection misuse, and internal classes holding external class references. The discovery methods include observing GC logs, monitoring using VisualVM or JConsole, generating HeapDump analysis, and positioning using Profiling tools. The troubleshooting steps are to check memory overflow errors, monitor memory trends, generate Dump files, analyze object distribution and GCRoots paths. It is recommended to clean the cache structure regularly, log out the listener in a timely manner, avoid the infinite growth of static collections, use non-static internal classes with caution, and use weak and soft references rationally.

Jul 15, 2025 am 03:09 AM

How to implement a binary search in Java?

BinarysearchinJavarequirescarefulhandlingofboundariesandconditionstoensurecorrectnessandefficiency.1.Useleft (right-left)/2topreventintegeroverflowwhencalculatingthemidpoint.2.Maintaintheloopconditionwhile(left

Jul 15, 2025 am 03:08 AM

Java for loop examples

There are three common forms of Java for loops. 1. The basic for loop is suitable for cases where the number of loops is known. The syntax is for (initialization; conditional judgment; update), such as traversing arrays or counts; 2. The enhanced for loop (for-each) is used to simplify the traversal of arrays or collections, and the syntax is for (element type variable name: the object to be traversed), but the index cannot be accessed or the collection content cannot be modified; 3. Nested for loops are used to deal with two-dimensional structures such as matrices, outer control rows, and inner control columns, but performance issues need to be paid attention to.

Jul 15, 2025 am 03:07 AM

java cycle

What are lambda expressions?

Lambda expressions are a way to write shorter, concise functions, especially for scenarios where simple functions are temporarily used. It is an anonymous function, often used to pass functions as parameters to other higher-order functions (such as map(), filter(), or sorted()), and can also be used to avoid defining complete functions for one-time logic. For example, in Python: double=lambdax:x*2 is equivalent to a simple function definition. 1. Main uses include: passing as parameters to higher-order functions; one-time logical encapsulation; quick binding in GUI or event processing. 2. Common use cases include: sorting lists based on custom keys, filtering data in real time, and transforming operations in data pipelines. 3. Make

Jul 15, 2025 am 03:00 AM

programming

What are generics in Java and why use them?

GenericsinJavasolvetheproblemoftypesafetyandeliminateruntimeClassCastExceptionsbyenforcingcompile-timetypechecks.Beforegenerics,collectionscouldholdanyobjecttype,forcingdeveloperstomanuallycastobjectswhenretrievingthem,whichcouldleadtoruntimeerrors.G

Jul 15, 2025 am 03:00 AM