API

String

  • str.repeate(count): String (J11)
  • str.isBlank(): Boolean (J11)
  • str.lines(): Stream<String> (J11)

I/O

  • Files (J7)
    • Files.readAllBytes(Path): byte[]
    • Files.readAllLines(Path, Charset): List<String>
    • try(Stream<String> lines = Files.lines(Path)) { ... } (J8)
    • Files.createDirectories(Path)
    • Files.copy(src:Path, dst:Path, StandardCopyOption.REPLACE_EXISTING)
    • Files.move(src:Path, dst:Path, StandardCopyOption.REPLACE_EXISTING)
    • Files.newBufferedReader(Path, Charset): BufferedReader
    • Files.newBufferedWriter(Path, Charset): BufferedWriter
    • Files.newInputStream(Path): InputStream
    • Files.newOutputStream(Path [, CREATE, APPEND]): OutputStream
    • Files.write(Path, byte[])
    • Files.list(Path): Stream<Path> (J8)
    • Files.walk(path).filter(path-> ).forEach(path-> ) (J8)
    • Files.setPosixFilePermissions(Path, PosixFilePermissions.fromString("rw-------"))

RegEx

  • (...) group with index
  • (?<GRPNAME>...) group with GRPNAME identifier (no space!)
  • var alert = "12:23".replaceAll("(?<H>[0-2]\\d):(?<M>[0-5]\\d)", "it happened at hour=${H} & minute=${M}")

Java 8 Time

Java 8 Class Function
java.time.Instant Moment in UTC
java.time.ZonedDate Moment with time zone
java.time.OffsetDateTime Moment with offset-from-UTC
java.time.LocalDateTime Date & Time of day (no offset, no zone)
java.time.LocalDate Date only (no offset, no zone)
java.time.LocalTime Time of day only (no offset, no zone)
java.time.format.DateTimeFormatter Formatter for printing and parsing date-time objects

Following graph shows java.util.Date, and other java.time.* classes conversions.

Java8DateTime

Concurrency

  • Visibility Problem (Jenkov & Baeldung)
    • A variable is cached in CPU’s cache and its change is not visible to other thread(s) running by other cores.
    • With non-volatile variables there are no guarantees about when the JVM reads data from main memory into CPU caches, or writes data from CPU caches to main memory.
    • Solutions
      • Use volatile modifier
        • mark a variable as “being stored in main memory”, not from the CPU cache
        • every write will be written to main memory
        • guarantees visibility of changes to variables across threads without providing mutual exclusion
      • Use Atomic* type for primitive types (they have volatile variable inside!)
      • Variables in synchronized blocks or methods
  • Race Condition: updating shared variable by concurrent threads. Solutions
    • Confinement: Don’t share
    • Immutability: Immutable data structures
    • Locking: Block other tasks to alter shared variables

A demonstration for volatile [REF]

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26

static boolean simple = true;
static volatile boolean vol = true;

public static void main(String[] args) throws Exception {
  var volTh = new Thread(() -> {
    while (vol) {}
  });

  var simpleTh = new Thread(() -> {
    while (simple) {}
  });

  volTh.start();
  simpleTh.start();

  Thread.sleep(1000);

  vol = false;
  simple = false;

  volTh.join();
  System.out.println("'vol' is false");    // This line is printed

  simpleTh.join();
  System.out.println("'simple' is false"); // This line may not be printed!
}

Thread-safe Singleton

  • [REF]
  • Lazy initialization
  • Using synchronized block to prevent race condition
  • Using double-checked locking to prevent recreation by different threads
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

public final class ThreadSafeSingleInstance {

    private static volatile ThreadSafeSingleInstance INSTANCE = null;

    private ThreadSafeSingleInstance() {}

    public static ThreadSafeSingleInstance getInstance() {
        if (INSTANCE == null) {
            synchronized(ThreadSafeSingleInstance.class) {
                if (INSTANCE == null) {
	                INSTANCE = new ThreadSafeSingleInstance();
                }
            }
        }
        return instance;
    }
}

JDBC

Driver Class JDBC URL Hibernate Dialect Maven Artifact
org.h2.Driver jdbc:h2:mem:test[;Mode=Oracle]
jdbc:h2:~/test[;Mode=Oracle]		 H2Dialect Maven Central
org.hsqldb.jdbc.JDBCDriver jdbc:hsqldb:mem:test[;sql.syntax_ora=true] HSQLDialect Maven Central
oracle.jdbc.driver.OracleDriver jdbc:oracle:thin:@//SERVER:1521/SERVICE
jdbc:oracle:thin:@SERVER:1521:SID		 Oracle12cDialect Maven Central
[ojbc8]
com.mysql.cj.jdbc.Driver jdbc:mysql://SERVER:3306/DATABSE[?useUnicode=true&characterEncoding=UTF-8] MySQL57Dialect or MySQL8Dialect Maven Central
[All]

Note: org.hibernate.dialect is Hibernate package for dialects.

Maven

  • Some options
    • -P PROFILE1[,PROFILE2,...]
    • -U - Forces a check for missing releases and updated snapshots on remote repositories
    • -ntp - Do not display transfer progress when downloading or uploading
  • Use a private registry, create/modify $HOME/.m2/settings.xml: 
    1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

    <?xml version="1.0" encoding="UTF-8"?>
<settings xmlns="http://maven.apache.org/SETTINGS/1.1.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.1.0 http://maven.apache.org/xsd/settings-1.1.0.xsd">
  
  <mirrors>
    <mirror>
      <id>central</id>
      <name>central</name>
      <url>http://NEXUS/repository/maven-public/</url>
      <mirrorOf>*</mirrorOf>
    </mirror>
  </mirrors>
  
</settings>
  • Java version (e.g. 1.8, 11), and other properties 
    1
2
3
4
5

    <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
</properties>