How to Set System Property

Java system properties and environment variables

I think the difference between the two boils down to access. Environment variables are accessible by any process and Java system properties are only accessible by the process they are added to.

Also as Bohemian stated, env variables are set in the OS (however they 'can' be set through Java) and system properties are passed as command line options or set via setProperty().

How to add new System Properties in java

Either System.setProperty or use the -Dname=value flag when you start the JVM

how to set system property in spring boot application

Not good idea to set system variables from inside Java code.
Basically, variables are meant to keep the code free from having any variable values.

Use properties files to store your configurations. Spring Boot does a great job externalising your configurations.
It also let to you have environmental configurations in separate files and does a great job initialising it.

Refer to https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html

Use of SystemPropertyInitializer to set System Property before setting property placeholder

The easiest solution would be to pass in the environment property as a command-line argument, so it can be resolved as a system property.

If that's not an option you can implement a ApplicationContextInitializer that promotes environment properties to system properties.

public class EnvironmentPropertyInitializer implements 
                   ApplicationContextInitializer<ConfigurableApplicationContext> {

    boolean override = false; //change if you prefer envionment over command line args

    @Override
    public void initialize(final ConfigurableApplicationContext applicationContext) {
        for (Entry<String, String> environmentProp : System.getenv().entrySet()) {
            String key = environmentProp.getKey();
            if (override || System.getProperty(key) == null) {
                System.setProperty(key, environmentProp.getValue());
            }
        }
    }
}

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Here it looks like you're using Spring Batch Admin, so you can register your initializer with a slight addition to the web.xml file:

<context-param>
    <param-name>contextInitializerClasses</param-name>
    <param-value>org.your.package.EnvironmentPropertyInitializer</param-value>
</context-param>

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Adding Background Since a Comment Didn't Seem Sufficient: Here's the relevant classes and the order in which they are called/evaluated.

1. The ApplicationContextInitializer tells the Spring Application how to load an application context and can be used to set bean profiles, and change other aspects of the context. This gets executed before the context gets completely created.
2. The PropertyPlaceholderConfigurer is a BeanFactoryPostProcessor and calls postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory). This modifies the BeanFactory to allow for resolution of properties like ${my.property:some.default} when setting the properties of a bean as it is created by the BeanFactory.
3. The SystemPropertyInitializer implements InitializingBean and calls afterPropertiesSet(). This method runs after a bean is instantiated and the properties have been set.

So you're right that the SystemPropertyInitializer will not help here since it evaluates after the properties are set on the PropertyPlaceholderConfigurer. The ApplicationContextInitializer, however, will be able to promote those environment properties to system properties so they can be interpreted by the XML.

And one more note that I forgot to mention, one of the first declared beans will need to be:

 <context:property-placeholder/>

Though it seems redundant, it will allow your PropertyPlaceholderConfigurer bean to evaluate ${ENVIRONMENT:hsql} correctly by using the environment properties you just promoted.

How to set System property for a scalatest from sbt command line

Your third way almost works (if fork in Test := true is set). But you have to change into the project first:

sbt/sbt  '; project mllib; set javaOptions in Test  +="-Dtest.num.points=500000"; \
test-only org.apache.spark.mllib.optimization.LBFGSSuite'

Set system property in jmockit unit test

Make sure you call System.setProperty("property.name", "https://justatest.com/dav/bulk"); before the class being tested is instantiated, otherwise the static field will always be null.

Consider using a @BeforeClass setup method for this:

@BeforeClass
public static void setup() {
    System.setProperty("property.name", "https://justatest.com/dav/bulk");
    // Instantiate your CsvConfig instance here if applicable.
}

and then

@Test
public void getUrl(){
    System.setProperty("property.name", "https://justatest.com/dav/bulk");
    String fileUrl = csvConfig.getFileUrl();
    assertEquals(fileUrl, "https://justatest.com/dav/bulk/otherstuff");
}

How to pass the -D System properties while testing on Eclipse?

Run -> Run configurations, select project, second tab: “Arguments”. Top box is for your program, bottom box is for VM arguments, e.g. -Dkey=value.

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