Value Loaders

strong.valid.simple Application Configuration

During AndHow's initialization, AndHow attempts to load values for each configuration property in your application. By default, AndHow uses its standard list of Loaders to load values from various configuration sources, such as JNDI, properties files or system properties. AndHow sequentially calls load() on each Loader and values are set for each Property on a first win basis, meaning that the first loader to find a non-null value for a Property sets its value. Values found later by other loaders farther down the list are ignored.

Below is the standard list of configuration sources and loaders, in order, from first to last:

Fixed values - values explicitly set in code during initiation - StdFixedValueLoader
String[] arguments from the static void main() method - StdMainStringArgsLoader
System Properties read from System.getPropertiesa() - StdSysPropLoader
Environmental Variables read from System.getenv() - StdEnvVarLoader
JNDI context variables - StdJndiLoader
Java properties file on the filesystem (path spec'ed as an AndHow property) - StdPropFileOnFilesystemLoader
Java properties file on the classpath (defaults to /andhow.properties) - StdPropFileOnClasspathLoader

The default set and order of loaders will work for most applications, but it's easy to change the order or insert other loaders into the the load sequence - there is an example of that at the bottom of this page.

In general you do not directly work with the Standard loaders because they are internally created by AndHow. Instead, the StdConfig API is used to configure how AndHow creates the standard loaders.

The Standard Loaders in detail, in Order

StdFixedValueLoader - Fixed values loader

The fixed value loader is used to set values directly in code.

Typical Use Case

Since this loader receives values directly assigned from your code, it is only useful for creating application test configurations. Configuration values can be set directly prior to running a test so that your application can be tested in a specific configured state. Since this is the first loader, a property value set by this load will override configuration values found by any other loader.

Basic Behaviors

Pre-trims String values: No
Complains about unrecognized properties: NA - This is not possible
Default behavior: None - This loader is only active if values are directly set as shown below

Loader Details and Configuration

One simple way to set fixed values is shown below. AndHow will discover this class implementing the AndHowInit interface during initialization to read your configuration:

import org.yarnandtail.andhow.*;

import org.yarnandtail.andhow.property.StrProp;

public class SetFixedValues implements AndHowInit {

@Override public AndHowConfiguration getConfiguration() {

return StdConfig.instance()

.addFixedValue(MY_PROP, "some value"); //MY_PROP is some visible property

}

Alternatively you can use AndHow.findConfig() at an application entry point such as the main method.

TODO: This example should be for a test, since that is the only appropriate usage for this loader.

StdMainStringArgsLoader - String[] arguments loader

Reads an array of Strings containing key value pairs in the form key=value, and loads the value for any key that matches a Property. This is normally used to read in command line arguments.

Typical Use Case

An executable jar or desktop application accepts command line arguments at startup to configure the application. This loader loads the String[] args passed to the main(String[] args), which Java passes from the command line. This is the most common usage, however, your application may retrieve an array of String's from anywhere to pass to AndHow at startup.

Basic Behaviors

Pre-trims String values: Yes
Complains about unrecognized properties: No
Default behavior: None - This loader is only active if command line arguments are passed in as shown below

Loader Details and Configuration

AndHow is not 'there' to somehow intercept command line arguments - Your application code will need to do that and pass those arguments to AndHow, like this:

import org.yarnandtail.andhow.*;

public class MyAppClass {

public static void main(String[] args) {

AndHow.findConfig()

.setCmdLineArgs(args)

.build();

}

Passing values from the command line looks like this:

java -jar MyJarName.jar full.name.of.MY_PROPERTY=someValue

Here is a complete example of using command line arguments.

StdSysPropLoader - System Properties Loader

Reads the Java system properties and loads the value for any system property who's name matches a Property.

Typical Use Case

An application might receive all or some of its configuration from Java system properties that are set when the JVM starts. Java system properties can be set in a startup script, which could be customized for each environment. This provides a relatively easy way for deployment automation or system administrators to control application configuration values across many servers.

Basic Behaviors

Pre-trims String values: No (Individual Properties may still trim values)
Complains about unrecognized properties: No
Default behavior: AndHow always attempts to read Java system properties and will assign property values if any of the system properties match known property names.

Loader Details and Configuration

This loader loads properties from java.lang.System.getProperties(). Over the lifecycle of the JVM, values of system properties can change so this loader is working from a snapshot of the system properties it finds at the time of AndHow initialization. Once loaded, AndHow property values never change.

For FlgProp properties (true/false flags), the StdSysPropLoader will set the Property's value to true if a matching environment variable is found, even if the value of the property is empty. System properties can be cleared via java.lang.System.clearProperty(name), which is how a flag value could be unset prior to AndHow loading.

Passing system properties on command line looks like this:

java -Dfull.name.of.MY_PROPERTY=someValue -jar MyJarName.jar

StdEnvVarLoader - Environmental Variables read from System.getenv()

Reads the operating system defined environment variables and loads the value for any environmental variable who's name matches a Property.

Typical Use Case

An application might receive all or some of its configuration from OS defined environmental variables that are passed to the JVM when the JVM starts. Environment variables can be set in the OS and augmented in a startup script, which could be customized for each environment. Similar to Java system properties, this provides a relatively easy way for deployment automation or system administrators to control application configuration values across many servers.

Basic Behaviors

Pre-trims String values: No (Individual Properties may still trim values)
Complains about unrecognized properties: No
Default behavior: AndHow always attempts to read environment variables and will assign property values if any of them match known property names.

Loader Details and Configuration

This loader loads values from System.getenv(). Those environmental values are provided by the host environment (the OS) as a static snapshot to the JVM at startup as a String-to-String map. The underlying OS environment variables can change, however, the JVM is unaware of it. Similarly, AndHow property values never change once loaded.

For FlgProp properties (true/false flags), the StdEnvVarLoader will set the Property's value to true if a matching environment variable is found, even if the value of the property is empty.

The Windows OS uses ALL CAPS for environment variables, while all others OS's are case sensitive - This is the primary reason AndHow is case insensitive by default. Each OS has a different way to set an environmental variables.

StdJndiLoader - JNDI context variables

Attempts to look up the name of each known Property in the JNDI environment and loads the value for any that are found.

Typical Use Case

A web or service application runs in an application container such as Tomcat. Application containers provide a JNDI environment which can be used to configure applications running in their environment.

Basic Behaviors

Pre-trims String values: No (Individual Properties may still trim values)
Complains about unrecognized properties: No
Default behavior: Always attempts to look up each Property in the JNDI environment.
Complains about missing JNDI environment: No (by default)
Is case sensitive: Yes (This is one of the only loaders that is case sensitive)

Loader Details and Configuration

While most other loaders are case insensitive, the JNDI loader is case sensitive because the JNDI API is case sensitive. Also, while most other loaders consume a configuration resource (e.g. a properties file) and read all the names and values, the JNDI loader works the other way: It goes through the list of known Properties looks up each property name in the JNDI context. This because its not possible to somehow read the entire JNDI environment.

JNDI implementations vary in how they name properties, so the loader will try several common name forms, for example, the JNDI loader will attempt to look up the following JNDI names for a property named org.foo.My_Prop:

- java:comp/env/org/foo/My_Prop
- java:comp/env/org.foo.My_Prop
- java:org/foo/My_Prop
- java:org.foo.My_Prop
- org/foo/My_Prop
- org.foo.My_Prop

This list has two different styles of property names: dot separated 'AndHow style' and slash separated JNDI style. Additionally, AndHow looks for three different roots (the part that comes before the variable name): java:comp/env/ is used by Tomcat and several other application servers, java: is used by some non-container environments and at least one application server (Glassfish) uses no root at all. In all, AndHow will search for each property under six different names (2 X 3). AndHow will throw an error at startup if it finds multiple names in the JNDI context that refer to the same property.

Specifying JNDI environment variables varies by environment, but here is an example of specifying some properties in a Tomcat context.xml file:

. . .

. . .

</Context>

In the example above, Tomcat will automatically prepend java:comp/env/ to the name it associates with each value. As the example shows, JNDI values can be typed. If AndHow finds the value to already be the type it expects (e.g. an Integer), great! If AndHow finds a String and needs a different type, AndHow will do the conversion. Any other type of conversion (e.g. from a Short to an Integer) will result in an exception.

If your JNDI environment uses a non-default different root, it can be added using one of the built-in Properties for the JNDI loader. Those property values would need to be loaded prior to the JNDI loader, so using system properties, for example, would work. Here is an example of adding the custom JNDI root java:xyz/ as a system property on command line:

java -Dorg.yarnandtail.andhow.load.std.StdJndiLoader.CONFIG.ADDED_JNDI_ROOTS=java:xyz/ -jar MyJarName.jar

StdPropFileOnFilesystemLoader - Java properties file on the filesystem

Parses and loads Properties from a Java .property file on the file system. Since file systems vary, there is no default filepath that AndHow attempts to load from.

Typical Use Case

A service application running in a web container (Tomcat or other) might load minimal configuration from system properties or environmental variables, loading the bulk of its configuration from a properties file on the file system. The properties file is not part of the application, so it survives redeployments. A single environmental or system property could then be used to specify the properties file.

Basic Behaviors

Pre-trims String values: No (Individual Properties may still trim values)
Complains about unrecognized properties: Yes
Complains if the .properties file is missing: Yes (if a file path is configured)
Default behavior: None - If not explicitly configured, no default loading is attempted.

Loader Details and Configuration

This loader reads properties files using the java.util.Properties, which silently ignores duplicate property entries (i.e., the same key appearing multiple times). When there are duplicate property keys in a properties file, only the last assigned value is used. Full details on how Java parses properties files can be found in the properties file specification.

This loader is only active if it is configured as shown below:

import org.yarnandtail.andhow.*;

import org.yarnandtail.andhow.property.StrProp;

public class UsePropertyFileOnFilesystem implements AndHowInit {

public static final StrProp MY_FILEPATH = StrProp.builder()

.desc("Path to a properties file on the file system. "

+ "If a path is configured, startup will FAIL if the file is missing.").build();

@Override

public AndHowConfiguration getConfiguration() {

return StdConfig.instance()

.setFilesystemPropFilePath(MY_FILEPATH);

}

The code above adds the property MY_FILEPATH (the name is arbitrary) which is used to configure the StdPropFileOnFilesystemLoader with a file location. When AndHow initializes, the StdPropFileOnFilesystemLoader checks to see if a value has been loaded for MY_FILEPATH by any prior loader. If a value is present, the loader tries to load from the configured file system path. If no value is configured, this loader is skipped.

StdPropFileOnClasspathLoader - Java properties file on the classpath

Parses and loads Properties from a Java .property file on the classpath. By default, this loader will look for a file named andhow.properties at the root of the classpath.

Typical Use Case

A service application might load the majority of its configuration from system properties or environmental variables, however, some sane default configuration values can be bundled with the application. By default, AndHow will discover and load a file named andhow.properties at the root of the classpath via the StdPropFileOnClasspathLoader.

Basic Behaviors

Pre-trims String values: No (Individual Properties may still trim values)
Complains about unrecognized properties: Yes
Complains if the .properties file is missing: No
Default behavior: Attempts to read andhow.properties from the root of the classpath

Loader Details and Configuration

Configuring the name or classpath of the properties file can be used to enable different configuration profiles based on the environment. For instance, a system property could specify that /test.properties be used on a test server and /production.properties on a production server. An example of configuring the property file path:

import org.yarnandtail.andhow.*;

import org.yarnandtail.andhow.property.StrProp;

public class UsePropertyFileOnClasspath implements AndHowInit {

public static final StrProp MY_CLASSPATH = StrProp.builder()

.desc("Path to a properties file on the classpath. "

+ "If the file is not present, it is not considered an error.").build();

@Override

public AndHowConfiguration getConfiguration() {

return StdConfig.instance()

.setClasspathPropFilePath(MY_CLASSPATH);

}

The code above adds the property MY_CLASSPATH (the name is arbitrary) which is used to configure the StdPropFileOnClasspathLoader with a custom property file location. When AndHow initializes, the StdPropFileOnClasspathLoader checks to see if a value has been loaded for MY_CLASSPATH by any prior loader. If a value is present, the loader tries to load from the configured classpath. If no value is configured, the default classpath is assumed.

Adding and Changing the Loader Order

It's easy to edit or change the order of the loaders:

package org.example;

import org.yarnandtail.andhow.*;

import org.yarnandtail.andhow.load.std.*;

public class ModifyStdLoaderOrder implements AndHowInit {

@Override public AndHowConfiguration getConfiguration() {

return StdConfig.instance()

.setStandardLoaders(StdEnvVarLoader.class, StdJndiLoader.class);

}

The example class above implements the AndHowInit interface. At startup, AndHow will discover this class and use it to configure itself. In this case, the standard list of seven loaders has been replaced with just two. Its also possible to leave the standard list intact and insert loaders between the standard loaders:

@Override public AndHowConfiguration getConfiguration() {

PropFileOnClasspathLoader pfl = new PropFileOnClasspathLoader();

pfl.setFilePath("/my.properties");

return StdConfig.instance()

.insertLoaderBefore(StdJndiLoader.class, pfl);

}

Only a single instance of each of the Standard loaders is allowed - these are the loaders with names start with 'Std' and implement the StandardLoader interface, however, it is OK to insert any number of any other loader.