In a previous post we learned that configuration property values can be passed via environment variables. With Spring Boot we can also pass the values using Java system properties. So if we have a property sample.message then we can use -Dsample.message=value to pass a value when we run the application. If we use the Spring Boot Gradle plugin we must reconfigure the bootRun task to pass Java system properties from the command-line.
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Spring Boot has many options for externalising the configuration of our application. One of the options it to use OS environment variables. We need to follow certain rules to name the environment variable and then Spring Boot will use the value of variable to set a configuration property. The property name must be in uppercase and any dots need to be replaced with underscores. For example the configuration property sample.message is set with the environment variable SAMPLE_MESSAGE. This feature can be useful in a continuous integration environment where we can set environment variables or just when developing locally. The nice thing is that this also works when we use the Spring Boot Gradle plugin. The environment variables are passed on to the Java process that the bootRun task starts.
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The auto-configuration feature in Spring Boot adds beans to our application context based on conditions. For example based on the availability of a class on the class path or a environment property beans are enabled or disabled. We must apply the @EnableAutoConfiguration or @SpringBootApplicaiton in our Spring Boot application to make this work. To get an overview of all the configurations that had positive and negative conditional matches in our application we can use the --debug command-line option. This will print out a report to System.out with a complete overview. We can check why a configuration is applied or not.
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When we develop a Spring Boot application we can hot reload newly compiled classes using Gradle. The bootRun task of the Spring Boot Gradle plugin has support for Spring Loaded. We must add a dependency to Spring Loaded as a dependency for the classpath configuration in the buildscript block. If we now use the bootRun task everything is ready to reload changed classes. Now we can use the continuous build feature of Gradle to listen for changes in our source files to recompile them. The recompiled classes are then reloaded in the running Spring Boot application started with bootRun. We start a second Gradle instance with the -t option for the classes task. This way when we change a source file it gets recompiled automatically and reloaded.
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In Spring we can use the @Value annotation to set property or arguments values based on a SpEL expression. If we want to use the @Value annotation for a constructor argument we must not forget to add the @Autowired annotation on the constructor as well.
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