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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​

tip

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.