Add enum parsing support to ConfigUtil

Makes it slightly easier to parse enumeration values, which currently
is only the LoginType under auth.type, but might be extended in the
future to additional values.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/server/config/AuthConfig.java

@@ -77,16 +77,7 @@ public class AuthConfig {
     if (isBecomeAnyoneEnabled()) {
       return LoginType.DEVELOPMENT_BECOME_ANY_ACCOUNT;
     }
-    String type = cfg.getString("auth", null, "type");
-    if (type == null) {
-      return LoginType.OPENID;
-    }
-    for (LoginType t : LoginType.values()) {
-      if (type.equalsIgnoreCase(t.name())) {
-        return t;
-      }
-    }
-    throw new IllegalStateException("Unsupported auth.type: " + type);
+    return ConfigUtil.getEnum(cfg, "auth", null, "type", LoginType.OPENID);
   }
 
   private static boolean isBecomeAnyoneEnabled() {

src/main/java/com/google/gerrit/server/config/ConfigUtil.java

@@ -18,9 +18,91 @@ import static org.spearce.jgit.util.StringUtils.equalsIgnoreCase;
 
 import org.spearce.jgit.lib.Config;
 
+import java.lang.reflect.InvocationTargetException;
 import java.util.concurrent.TimeUnit;
 
 public class ConfigUtil {
+  /**
+   * Parse a Java enumeration from the configuration.
+   *
+   * @param <T> type of the enumeration object.
+   * @param config the configuration file to read.
+   * @param section section the key is in.
+   * @param subsection subsection the key is in, or null if not in a subsection.
+   * @param setting name of the setting to read.
+   * @param defaultValue default value to return if the setting was not set.
+   *        Must not be null as the enumeration values are derived from this.
+   * @return the selected enumeration value, or {@code defaultValue}.
+   */
+  @SuppressWarnings("unchecked")
+  public static <T extends Enum<?>> T getEnum(final Config config,
+      final String section, final String subsection, final String setting,
+      final T defaultValue) {
+    final T[] all;
+    try {
+      all = (T[]) defaultValue.getClass().getMethod("values").invoke(null);
+    } catch (IllegalArgumentException e) {
+      throw new IllegalArgumentException("Cannot obtain enumeration values", e);
+    } catch (SecurityException e) {
+      throw new IllegalArgumentException("Cannot obtain enumeration values", e);
+    } catch (IllegalAccessException e) {
+      throw new IllegalArgumentException("Cannot obtain enumeration values", e);
+    } catch (InvocationTargetException e) {
+      throw new IllegalArgumentException("Cannot obtain enumeration values", e);
+    } catch (NoSuchMethodException e) {
+      throw new IllegalArgumentException("Cannot obtain enumeration values", e);
+    }
+    return getEnum(config, section, subsection, setting, all, defaultValue);
+  }
+
+  /**
+   * Parse a Java enumeration from the configuration.
+   *
+   * @param <T> type of the enumeration object.
+   * @param config the configuration file to read.
+   * @param section section the key is in.
+   * @param subsection subsection the key is in, or null if not in a subsection.
+   * @param setting name of the setting to read.
+   * @param all all possible values in the enumeration which should be
+   *        recognized. This should be {@code EnumType.values()}.
+   * @param defaultValue default value to return if the setting was not set.
+   *        This value may be null.
+   * @return the selected enumeration value, or {@code defaultValue}.
+   */
+  public static <T extends Enum<?>> T getEnum(final Config config,
+      final String section, final String subsection, final String setting,
+      final T[] all, final T defaultValue) {
+    final String valueString = config.getString(section, subsection, setting);
+    if (valueString == null) {
+      return defaultValue;
+    }
+
+    String n = valueString.replace(' ', '_');
+    for (final T e : all) {
+      if (equalsIgnoreCase(e.name(), n)) {
+        return e;
+      }
+    }
+
+    final StringBuilder r = new StringBuilder();
+    r.append("Value \"");
+    r.append(valueString);
+    r.append("\" not recognized in ");
+    r.append(section);
+    if (subsection != null) {
+      r.append(".");
+      r.append(subsection);
+    }
+    r.append(".");
+    r.append(setting);
+    r.append("; supported values are: ");
+    for (final T e : all) {
+      r.append(e.name());
+      r.append(" ");
+    }
+    throw new IllegalArgumentException(r.toString().trim());
+  }
+
   /**
    * Parse a numerical time unit, such as "1 minute", from the configuration.
    *