Accessing Spring properties in a non-Spring application. This test demonstrates creating an application context that doesn't create beans but does setup the environment.

SpringPropertiesTest.java

package com.example;

import java.util.Arrays;
import java.util.NavigableSet;
import java.util.Set;
import java.util.TreeSet;

import org.junit.Assert;
import org.junit.Test;
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.Banner;
import org.springframework.boot.SpringApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.EnumerablePropertySource;
import org.springframework.core.env.Environment;
import org.springframework.core.env.PropertySource;

public class SpringPropertiesTest {
    private final static Class<?> APPLICATION_CLASS = SpringPropertiesTest.class;
    private final static String[] NO_ARGS = { };
    
    @Test
    public void testCommandLineProperty() {
        int port = 8081;
        String[] args = { "--port=" + port };
        Environment environment = getEnvironment(APPLICATION_CLASS, args);
        
        // Retrieve property as both string and int.
        Assert.assertEquals(Integer.toString(port), environment.getProperty("port"));
        Assert.assertEquals(port, environment.getProperty("port", Integer.class).intValue());
    }
    
    @Test
    public void testStandardKey() {
        Environment environment = getEnvironment(APPLICATION_CLASS, NO_ARGS);
        
        Assert.assertTrue(environment.containsProperty("spring.beaninfo.ignore"));
    }
    
    @Test
    public void testGetAllKeys() {
        Environment environment = getEnvironment(APPLICATION_CLASS, NO_ARGS);
        Set<String> keys = getAllKeys(environment);
        
        Assert.assertTrue(keys.contains("java.version"));
    }
    
    @Test
    public void testSystemProperty() {
        Environment environment = getEnvironment(APPLICATION_CLASS, NO_ARGS);
        String systemPropertyKey = "java.version";
        
        // Test that system properties are available via Spring environment.
        Assert.assertEquals(System.getProperty(systemPropertyKey), environment.getProperty(systemPropertyKey));
    }
    
    private static Environment getEnvironment(Object source, String[] args) {
        SpringApplication springApplication = new BeanlessSpringApplication(source);
        ApplicationContext context = springApplication.run(args);
        
        return context.getEnvironment();
    }
    
    private static NavigableSet<String> getAllKeys(Environment environment) {
        NavigableSet<String> allKeys = new TreeSet<>();
        
        for (PropertySource<?> propertySource : ((ConfigurableEnvironment)environment).getPropertySources()) {
            if (propertySource instanceof EnumerablePropertySource) {
                String[] keys = ((EnumerablePropertySource<?>)propertySource).getPropertyNames();
                
                allKeys.addAll(Arrays.asList(keys));
            }
        }
        
        return allKeys;
    }
    
    // Normal usage of BeanlessSpringApplication would be:
    //
    //   SpringApplication springApplication = new BeanlessSpringApplication(Application.class);
    //   ApplicationContext context = springApplication.run(args);
    //
    // Where "Application.class" is our main class and "args" are the arguments passed to its main method.
    //
    /** A SpringApplication subclass that does not create beans (the most expensive part of application startup). */
    public static class BeanlessSpringApplication extends SpringApplication {
        public BeanlessSpringApplication(Object... sources) {
            super(sources);
            
            setBannerMode(Banner.Mode.OFF);
        }
        
        @Override
        protected void refresh(ApplicationContext applicationContext) {
            // Do nothing.
        }
        
        @Override
        protected void afterRefresh(ConfigurableApplicationContext context, ApplicationArguments args) {
            // Do nothing.
        }
    }
}