On Windows, the key to send JSON data is double-quotes like this -d "{\"name\":\"Spring Forever\"}" cURL to POST a JSON data curl -X POST localhost:8080/books -H "Content-type:application/json" -d "{\"name\":\"Spring Forever\",\"author\":\"pivotal\"}" References cURL – Post JSON data to Spring REST
This java.lang.ArrayIndexOutOfBoundsException is thrown when we are accessing an array with an index which is greater than the size of an array. P.S Array index starts with 0 ArrayExample.java package com.mkyong; public class ArrayExample { public static void main(String[] args) { // array of 3 int[] num = new int[3]; num[0] = 1; num[1] = […]
Test a Spring Webflux endpoint with the WebTestClient, and hits the following error messages. Is this possible to increase the timeout? java.lang.IllegalStateException: Timeout on blocking read for 5000 MILLISECONDS at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:117) at reactor.core.publisher.Mono.block(Mono.java:1524) at org.springframework.test.web.reactive.server.ExchangeResult.formatBody(ExchangeResult.java:247) at org.springframework.test.web.reactive.server.ExchangeResult.toString(ExchangeResult.java:216) at java.base/java.lang.String.valueOf(String.java:2788) at java.base/java.lang.StringBuilder.append(StringBuilder.java:135) at org.springframework.test.web.reactive.server.ExchangeResult.assertWithDiagnostics(ExchangeResult.java:200) Solution By default, the WebTestClient will be timeout after 5 seconds. We […]
In Java, we can use ProcessBuilder to run a Windows batch file like this : ProcessBuilder processBuilder = new ProcessBuilder("C:\\Users\\mkyong\\hello.bat"); //or ProcessBuilder processBuilder = new ProcessBuilder(); processBuilder.command("cmd", "/c", "hello.bat"); File dir = new File("C:\\Users\\mkyong\\"); processBuilder.directory(dir); Alternatively, Runtime.getRuntime().exec like this : Process process = Runtime.getRuntime().exec( "cmd /c hello.bat", null, new File("C:\\Users\\mkyong\\")); 1. Java Example 1.1 A […]
Two Java examples to show you how to list files in a directory : For Java 8, Files.walk Before Java 8, create a recursive loop to list all files. 1. Files.walk 1.1 List all files. try (Stream<Path> walk = Files.walk(Paths.get("C:\\projects"))) { List<String> result = walk.filter(Files::isRegularFile) .map(x -> x.toString()).collect(Collectors.toList()); result.forEach(System.out::println); } catch (IOException e) { e.printStackTrace(); […]
In Python, we can use os.walker or glob to create a find() like function to search or list files or folders in a specified directory and also it’s subdirectories. 1. os.walker 1.1 List all .txt files in a specified directory + subdirectories. import os path = ‘c:\\projects\\hc2\\’ files = [] # r=root, d=directories, f = […]
In Java, we can use ProcessBuilder to call external commands easily : ProcessBuilder processBuilder = new ProcessBuilder(); // — Linux — // Run a shell command processBuilder.command("bash", "-c", "ls /home/mkyong/"); // Run a shell script processBuilder.command("path/to/hello.sh"); // — Windows — // Run a command processBuilder.command("cmd.exe", "/c", "dir C:\\Users\\mkyong"); // Run a bat file processBuilder.command("C:\\Users\\mkyong\\hello.bat"); Process […]
The Files.walk API is available since Java 8; it helps to walk a file tree at a given starting path. Topics Files.walk() method signature List all files List all folders or directories Find files by file extension Find files by filename Find files by file size Update all files last modified date 1. Files.walk() method […]
In Java, there are few ways to read a file line by line into a List 1. Java 8 stream List<String> result; try (Stream<String> lines = Files.lines(Paths.get(fileName))) { result = lines.collect(Collectors.toList()); } 2. Java 7 Files.readAllLines(new File(fileName).toPath(), Charset.defaultCharset()); 3. Classic BufferedReader example. List<String> result = new ArrayList<>(); BufferedReader br = null; try { br = […]
Python example to read a log file, line by line into a list. # With ‘\n’, [‘1\n’, ‘2\n’, ‘3’] with open(‘/www/logs/server.log’) as f: content = f.readlines() # No ‘\n’, [‘1’, ‘2’, ‘3’] with open(‘/www/logs/server.log’) as f: content = f.read().splitlines() 1. Read File -> List 1.1 A dummy log file. d:\\server.log a b c d 1 […]
Add –allow-unrelated-histories to solve the Git fatal error – “refusing to merge unrelated histories” Terminal $ git pull origin master From https://github.com/mkyong/java-concurrency * branch master -> FETCH_HEAD fatal: refusing to merge unrelated histories To fix it : Terminal $ git pull origin master –allow-unrelated-histories From https://github.com/mkyong/java-concurrency * branch master -> FETCH_HEAD Merge made by the […]
In Java, we can use BlockingQueue to create a queue which shared by both producer and the consumer. Producer – Generate data and put it into the queue. Consumer – Remove the data from the queue. Note Read this article to understand what is producer and consumer. The BlockingQueue implementations are thread-safe, safely be used […]
In Java, we can use String.contains() to check if a String contains a substring. 1. String.contains() – Case Sensitive JavaExample1.java package com.mkyong; public class JavaExample1 { public static void main(String[] args) { String name = "mkyong is learning Java 123"; System.out.println(name.contains("Java")); // true System.out.println(name.contains("java")); // false System.out.println(name.contains("MKYONG")); // false System.out.println(name.contains("mkyong")); // true if (name.contains("Java")) { […]
In Java, we can use Semaphore to limit the number of threads to access a certain resource. 1. What is Semaphore? In short, a semaphore maintains a set of permits (tickets), each acquire() will take a permit (ticket) from semaphore, each release() will return back the permit (ticket) back to the semaphore. If permits (tickets) […]
An example to show you how to create a thread safe sequence generator. 1. SequenceGenerator SequenceGenerator.java package com.mkyong.concurrency.examples.sequence.generator; public interface SequenceGenerator { long getNext(); } 1.1 First try, read, add, write the value directly. Below method is not thread safe, multiple threads may get the same value at the same time. UnSafeSequenceGenerator.java package com.mkyong.concurrency.examples.sequence.generator; public […]
In Java, we can use ExecutorService to create a thread pool, and tracks the progress of the asynchronous tasks with Future. The ExecutorService accept both Runnable and Callable tasks. Runnable – Return void, nothing. Callable – Return a Future. 1. ExecutorService 1.1 A classic ExecutorService example to create a thread pool with 5 threads, submit […]
In IDEA, clicks on the method name, press CTRL + Q to show the method signature on a pop up. Alternatively, press CTRL + P to show the available parameters.
In Java, we can use ScheduledExecutorService to run a task periodically or once time after a predefined delay by TimeUnit. 1. Run a Task Once The ScheduledExecutorService accepts both Runnable and Callable tasks. 1.1 Run a Runnable task after 5 seconds initial delay. ScheduledExecutorExample.java package com.mkyong.concurrency.executor.scheduler; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ScheduledExecutorRunnable […]
What is Fork/Join? Read this Java Fork/Join paper by Doug Lea The fork/join framework is available since Java 7, to make it easier to write parallel programs. We can implement the fork/join framework by extending either RecursiveTask or RecursiveAction 1. Fork/Join – RecursiveTask A fork join example to sum all the numbers from a range. […]
Fibonacci number – Every number after the first two is the sum of the two preceding. Few Java examples to find the Fibonacci numbers. 1. Java 8 stream 1.1 In Java 8, we can use Stream.iterate to generate Fibonacci numbers like this : Stream.iterate(new int[]{0, 1}, t -> new int[]{t[1], t[0] + t[1]}) .limit(10) .forEach(x […]
In Java 8, we can use Stream.iterate to create stream values on demand, so called infinite stream. 1. Stream.iterate 1.1 Stream of 0 – 9 //Stream.iterate(initial value, next value) Stream.iterate(0, n -> n + 1) .limit(10) .forEach(x -> System.out.println(x)); Output 0 1 2 3 4 5 6 7 8 9 1.2 Stream of odd numbers […]
In Git, we can use git reset –soft HEAD~1 to undo the last commit in local. (The committed files haven’t pushed to the remote git server) 1. Case Study git commit and find out some unwanted target/* files are committed accidentally, I haven’t issue the git push, any idea how to undo the commit? Terminal […]
In Git, we can use git reset HEAD — ‘files/folders/patterns’ to remove files or folders from the staging area (Changes to be committed). 1. Case Study git add some unwanted target/*.java files, still in the staging, haven’t committed yet. Terminal $ git add project/*/*.java $ git status On branch master No commits yet Changes to […]
In Git, we can use git show commit_id –name-only to list all the committed files that are going to push to the remote repository. P.S The git status didn’t show the committed files. 1. Case Study Below case, git add and committed too many unwanted target/* files accidentally, how to remove the committed files in […]
A JMH benchmark test about Forward loop vs Reverse loop for a List. There is a myth about reverse loop is faster, is this true? Tested with JMH 1.21 Java 10 Maven 3.6 CPU i7-7700 Forward loop for (int i = 0; i < aList.size(); i++) { String s = aList.get(i); System.out.println(s); } Reverse loop […]
Benchmark (N) Mode Cnt Score Error Units BenchmarkLoop.loopFor 10000000 avgt 10 61.673 ± 1.251 ms/op BenchmarkLoop.loopForEach 10000000 avgt 10 67.582 ± 1.034 ms/op BenchmarkLoop.loopIterator 10000000 avgt 10 66.087 ± 1.534 ms/op BenchmarkLoop.loopWhile 10000000 avgt 10 60.660 ± 0.279 ms/op In Java, we can use JMH (Java Microbenchmark Harness) framework to measure the performance of a […]
In Maven, you can use Apache Maven Dependency Plugin, goal dependency:purge-local-repository to remove the project dependencies from the local repository, and re-download it again. Terminal $ mvn dependency:purge-local-repository [INFO] Scanning for projects… [INFO] [INFO] ————–< com.mkyong.examples:maven-code-coverage >————— [INFO] Building maven-code-coverage 1.0-SNAPSHOT [INFO] ——————————–[ jar ]——————————— [INFO] [INFO] — maven-dependency-plugin:2.8:purge-local-repository (default-cli) @ maven-code-coverage — Downloading from […]
In this article, we will show you how to use Maven PMD Plugin to analyze the Java code. P.S PMD requires Java 1.7 1. Maven PMD Plugin Define the maven-pmd-plugin in the reporting tag, so that mvn site will generate the PMD report. pom.xml <reporting> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-pmd-plugin</artifactId> <version>3.11.0</version> </plugin> </plugins> </reporting> 2. Java […]
In Java, there is no global keyword, but we can use public static variable to referring a global variable. For example : MagicUtils.java package com.mkyong.example; public class MagicUtils { public static final String NAME = "mkyong"; // global public static final int LUCKY_NUMBER = 7; // global } JavaExample.java package com.mkyong.example; public class JavaExample { […]
In this article, we will show you how to use SpotBugs Maven Plugin to find bugs in Java code. Note Findbugs is no longer maintained, and thus SpotBugs is the spiritual successor of FindBugs P.S SpotBugs requires JDK 1.8 1. Maven SpotBugs Plugin Define the spotbugs-maven-plugin in the reporting tag. So that mvn site will […]
Generating a Maven report with mvn site, but hits the following errors java.lang.NoClassDefFoundError: org/apache/maven/doxia/siterenderer/DocumentContent Caused by: java.lang.ClassNotFoundException: org.apache.maven.doxia.siterenderer.DocumentContent [INFO] ———————————————————————— [INFO] BUILD FAILURE [INFO] ———————————————————————— [INFO] Total time: 28.280 s [INFO] Finished at: 2018-11-19T13:20:14+08:00 [INFO] ———————————————————————— [ERROR] Failed to execute goal org.apache.maven.plugins:maven-site-plugin:3.3:site (default-site) on project maven-static-code-analysis: Execution default-site of goal org.apache.maven.plugins:maven-site-plugin:3.3:site failed: A required class […]
In this article, we will show you how to use a Maven PIT mutation testing plugin to generate a mutation test coverage report for a Java project. Tested with Maven 3.5.3 JUnit 5.3.1 PITest 1.4.3 Note Line coverage tools like JaCoCo is just telling whether the code is tested or covered, while this PITest mutation […]
In this article, we will show you how to use a JaCoCo Maven plugin to generate a code coverage report for a Java project. Tested with Maven 3.5.3 JUnit 5.3.1 jacoco-maven-plugin 0.8.2 Note JaCoCo is an actively developed line coverage tool, that is used to measure how many lines of our code are tested. 1. […]
In Java, we can use Set.iterator().next() to get the first item from a java.util.Set JavaExample.java package com.mkyong; import java.util.HashSet; import java.util.Set; public class JavaExample { public static void main(String[] args) { Set<String> examples = new HashSet<>(); examples.add("1"); examples.add("2"); examples.add("3"); examples.add("4"); examples.add("5"); System.out.println(examples.iterator().next()); // java 8 System.out.println(examples.stream().findFirst().get()); } } Output 1 1 References Oracle doc – […]
On Debian 9.5, the MySQL package is still the old MySQL 5.5. This little guide show you how to upgrade it to MySQL 5.7 1. Download MySQL APT 1.1 Get the MySQL APT and select MySQL 5.7 $ wget https://dev.mysql.com/get/mysql-apt-config_0.8.10-1_all.deb $ sudo gdebi mysql-apt-config_0.8.10-1_all.deb 1.2 Below “configuration mysql-apt-config” screen will be prompted, clicks the first […]
In this article, we will show you how to upgrade from Debian 7 (wheezy) to Debian 9 (stretch) 1. Debian code name First, understand the Debian code name, read this Debian releases Debian 9 (stretch) Debian 8 (jessie) Debian 7 (wheezy) Debian 6 (squeeze) P.S On Debian 7, first upgrade to Debian 8, then Debian […]
Upgraded a Nginx Server to the latest 1.15.x, but hits the following error message : Terminal $ sudo service nginx status * nginx.service Loaded: masked (/dev/null; bad) Active: inactive (dead) $ sudo service nginx restart Failed to restart nginx.service: Unit nginx.service is masked. Solution To solve it, just unmask with this : sudo systemctl unmask […]
In Java, we can use BigInteger.valueOf(int) to convert int to a BigInteger JavaExample.java package com.mkyong; import java.math.BigInteger; public class JavaExample { public static void main(String[] args) { int n = 100; System.out.println(n); // convert int to Integer Integer integer = Integer.valueOf(n); System.out.println(integer); // convert int to BigInteger BigInteger bigInteger = BigInteger.valueOf(n); System.out.println(bigInteger); // convert Integer […]
The sum() method is available in the primitive int-value stream like IntStream, not Stream<Integer>. We can use mapToInt() to convert a stream integers into a IntStream. int sum = integers.stream().mapToInt(Integer::intValue).sum(); int sum = integers.stream().mapToInt(x -> x).sum(); Full example. Java8Stream.java package com.mkyong.concurrency; import java.util.Arrays; import java.util.List; import java.util.stream.Stream; public class Java8Stream { public static void main(String[] […]
In this article, we will show you few Maven profile examples to pass different parameters (server or database parameters) for different environments (dev, test or prod). P.S Tested with Maven 3.5.3 1. Basic Maven Profiles 1.1 A simple profile to skip the unit test. pom.xml <!– skip unit test –> <profile> <id>xtest</id> <properties> <maven.test.skip>true</maven.test.skip> </properties> […]
