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Java

Common directory for JDKs installed with DNF​

/usr/lib/jvm

Common Maven terminal commands​

Clean target dir and build project​

./mvnw clean install
tip

Option -DskipTests can be used to ignore the tests

Generate artifacts​

./mvnw package

Run the test cases​

./mvnw test
tip

To run only specific tests use option -Dtest=TestClassName

Show dependency tree​

./mvnw dependency:tree

Download sources​

./mvnw dependency:sources

List available updates​

./mvnw versions:display-dependency-updates
warning

If more than one version of Java is installed on the host a suitable one might need to be specified instead of the default before running the commands above, i.e.

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk
./mvnw...
note

If Maven is installed system-wide the mvn command can be used instead of the Maven Wrapper (mvnw)

Ivy​

Duplicate dependencies​

When a jar file is added to the classpath more than once (i.e. because it is a sub-dependency of multiple libraries that use different versions of it) the first one declared in ivy.xml takes precedence.

Mockito​

  • Full mocks (created with Mockito.mock()) don't execute target code during stubbing, so matchers never become actual arguments. Spies (Mockito.spy()) do execute real code, so matchers must be handled with doXxx().when() syntax instead of when().thenSomething() directly.

  • When using @InjectMocks objects mocked with @Mock need to have EXACTLY the same type in both the original and tests classes (not a subclass/superclass).

Run program in the terminal without manually compiling it first​

.java source code files​

java src/FileCounter.java

Shebang files​

Start the file of the Java program with

#!/usr/lib/jvm/java-18/bin/java --source 11

Then give it execution permission and run it with ./script (the .java extension is unnecessary). The --source option can be used to specify a different version of Java used by the program, although the target JRE needs to support it.

File counter example.

note
  • Target file should have a main method
  • Java 11+ required

Spring​

Common static imports used in tests​

import static org.assertj.core.api.Assertions.*;
import static org.hamcrest.Matchers.*;
import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.ArgumentMatchers.*;
import static org.mockito.BDDMockito.*;
import static org.mockito.Mockito.*;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

Default variable values can also be set in .properties files​

For example to default to the blank String when the environment variable is missing:

some.property=${SOME_VARIABLE:}

Useful cron expressions that can be used with @Scheduled​

Every second

* * * * * *

Every minute

0 0/1 * 1/1 * ?

String​

Comparisons (equals vs. ==)​

Generally .equals() needs to be used, so the contents are compared. If the argument is null it returns false.

With == whether both operands refer to the same object is checked.