feat: Load System properties from configuration file

Closes #254, #255

README.md

@@ -65,6 +65,15 @@ Next, point MCS to that trust store like so
 mcs -Djavax.net.ssl.trustStore=/path/to/keystore search something
 ```
 
+Or, even easier: create a directory **.mcs** in your user directory (typically **C:\Users\<your-user-name>** on 🪟, **/home/<your-user-name>** on 🐧 or **/Users/<your-user-name>** on 🍎).
+Inside that folder, create a file **mcs.config** and write the following line in it:
+
+```
+javax.net.ssl.trustStore=/path/to/keystore
+```
+
+This way, you don't have to remember passing the `-Djavax.net.ssl.trustStore=`.
+
 ## Contributing
 Probably the easiest way to get a working development environment is to use Gitpod:
 

src/main/java/it/mulders/mcs/App.java

@@ -2,6 +2,7 @@
 
 import it.mulders.mcs.cli.Cli;
 import it.mulders.mcs.cli.CommandClassFactory;
+import it.mulders.mcs.cli.SystemPropertyLoader;
 import it.mulders.mcs.common.McsExecutionExceptionHandler;
 import picocli.CommandLine;
 
@@ -12,6 +13,7 @@ public static void main(final String... args) {
 
     // Visible for testing
     static int doMain(final String... args) {
+        System.setProperties(new SystemPropertyLoader().getProperties());
         var cli = new Cli();
         var program = new CommandLine(cli, new CommandClassFactory(cli))
                 .setExecutionExceptionHandler(new McsExecutionExceptionHandler());

src/main/java/it/mulders/mcs/cli/SystemPropertyLoader.java

@@ -0,0 +1,48 @@
+package it.mulders.mcs.cli;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Properties;
+
+/**
+ * Loads additional system properties from a predefined file on disk.
+ * <br />
+ * Copies those properties over the existing ones @{{@link System#getProperties()}} so the result
+ * is a drop-in replacement that could be passed for {@link System#setProperties(Properties)}.
+ * This class does not modify the System properties itself.
+ */
+public class SystemPropertyLoader {
+    private static final Path MCS_PROPERTIES_FILE = Paths.get(
+            System.getProperty("user.home"),
+            ".mcs",
+            "mcs.config"
+    );
+
+    private final Properties properties;
+
+    public SystemPropertyLoader() {
+        this(MCS_PROPERTIES_FILE);
+    }
+
+    protected SystemPropertyLoader(final Path source) {
+        var input = new Properties();
+
+        if (Files.exists(source) && Files.isRegularFile(source)) {
+            try (var reader = Files.newBufferedReader(source)) {
+                input.load(reader);
+            } catch (IOException ioe) {
+                System.err.printf("Failed to load %s: %s%n", source, ioe.getLocalizedMessage());
+            }
+        }
+
+        this.properties = new Properties();
+        properties.putAll(System.getProperties());
+        properties.putAll(input);
+    }
+
+    public Properties getProperties() {
+        return properties;
+    }
+}

src/test/java/it/mulders/mcs/cli/SystemPropertyLoaderTest.java

@@ -0,0 +1,43 @@
+package it.mulders.mcs.cli;
+
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import org.assertj.core.api.WithAssertions;
+import org.junit.jupiter.api.DisplayNameGeneration;
+import org.junit.jupiter.api.DisplayNameGenerator;
+import org.junit.jupiter.api.Test;
+
+@DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
+class SystemPropertyLoaderTest implements WithAssertions {
+    private static final Path SAMPLE = Paths.get("src", "test", "resources", "sample-mcs.config");
+    @Test
+    void should_load_if_file_exists() {
+        var loader = new SystemPropertyLoader(SAMPLE);
+
+        assertThat(loader.getProperties())
+                .containsEntry("example.a", "foo");
+    }
+
+    @Test
+    void should_not_fail_if_file_does_not_exist() {
+        var loader = new SystemPropertyLoader(Paths.get("src", "test", "resources", "non-existing-mcs.config"));
+
+        assertThat(loader.getProperties().isEmpty()).isFalse();
+    }
+
+    @Test
+    void should_delegate_to_System_properties() {
+        var loader = new SystemPropertyLoader(SAMPLE);
+
+        // user.dir is not overridden in the sample configuration file
+        assertThat(loader.getProperties()).containsKey("user.dir");
+    }
+
+    @Test
+    void should_override_System_properties() {
+        var loader = new SystemPropertyLoader(SAMPLE);
+
+        // user.home is recklessly overridden in the sample configuration file
+        assertThat(loader.getProperties()).containsEntry("user.home", "whatever");
+    }
+}

src/test/resources/sample-mcs.config

@@ -0,0 +1,4 @@
+example.a=foo
+example.b=${example.a} bar
+example.c=${example.a} ${example.a} baz
+user.home=whatever