Add dynamic command availability

Introduce availability concept on MethodTarget (with reason if not available)
Add bridge to @CliAvailabilityIndicator to Legacy registrar

Fixes #138

Add help for unavailable commands

Add standard API for availability

spring-shell-core/src/main/java/org/springframework/shell/Availability.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.shell;
+
+import org.springframework.util.Assert;
+
+/**
+ * Indicates whether or not a command is currently available. When not available, provides
+ * a reason.
+ *
+ * @author Eric Bottard
+ */
+public class Availability {
+
+    private final String reason;
+
+    private Availability(String reason) {
+        this.reason = reason;
+    }
+
+    public static Availability available() {
+        return new Availability(null);
+    }
+
+    public static Availability unavailable(String reason) {
+        Assert.notNull(reason, "Reason for not being available must be provided");
+        return new Availability(reason);
+    }
+
+    public boolean isAvailable() {
+        return reason == null;
+    }
+
+    public String getReason() {
+        return reason;
+    }
+}

spring-shell-core/src/main/java/org/springframework/shell/CommandNotCurrentlyAvailable.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.shell;
+
+/**
+ * Indicates that a command exists but is currently not invokable.
+ *
+ * @author Eric Bottard
+ */
+public class CommandNotCurrentlyAvailable extends RuntimeException {
+
+    private final String command;
+    private final Availability availability;
+
+    public CommandNotCurrentlyAvailable(String command, Availability availability) {
+        super(String.format("Command '%s' exists but is not currently available because %s", command, availability.getReason()));
+        this.command = command;
+        this.availability = availability;
+    }
+
+    public String getCommand() {
+        return command;
+    }
+
+    public Availability getAvailability() {
+        return availability;
+    }
+}

spring-shell-core/src/main/java/org/springframework/shell/MethodTarget.java

@@ -19,6 +19,7 @@ package org.springframework.shell;
 import java.lang.reflect.Method;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.function.Supplier;
 
 import org.springframework.util.Assert;
 import org.springframework.util.ReflectionUtils;
@@ -36,7 +37,16 @@ public class MethodTarget {
 
 	private final String help;
 
+	/**
+	 * If not null, returns whether or not the command is currently available. Implementations must be idempotent.
+	 */
+	private final Supplier<Availability> availabilityIndicator;
+
 	public MethodTarget(Method method, Object bean, String help) {
+		this(method, bean, help, null);
+	}
+
+	public MethodTarget(Method method, Object bean, String help, Supplier<Availability> availabilityIndicator) {
 		Assert.notNull(method, "Method cannot be null");
 		Assert.notNull(bean, "Bean cannot be null");
 		Assert.hasText(help, String.format("Help cannot be blank when trying to define command based on '%s'", method));
@@ -44,6 +54,7 @@ public class MethodTarget {
 		this.method = method;
 		this.bean = bean;
 		this.help = help;
+		this.availabilityIndicator = availabilityIndicator != null ? availabilityIndicator : () -> Availability.available();
 	}
 
 	/**
@@ -51,13 +62,21 @@ public class MethodTarget {
 	 * in case of overloaded method.
 	 */
 	public static MethodTarget of(String name, Object bean, String help) {
+		return of(name, bean, help, null);
+	}
+
+	/**
+	 * Construct a MethodTarget for the unique method named {@literal name} on the given object. Fails with an exception
+	 * in case of overloaded method.
+	 */
+	public static MethodTarget of(String name, Object bean, String help, Supplier<Availability> availabilityIndicator) {
 		Set<Method> found = new HashSet<>();
 		ReflectionUtils.doWithMethods(bean.getClass(), found::add, m -> m.getName().equals(name));
 		if (found.size() != 1) {
 			throw new IllegalArgumentException(String.format("Could not find unique method named '%s' on object of class %s. Found %s",
 				name, bean.getClass(), found));
 		}
-		return new MethodTarget(found.iterator().next(), bean, help);
+		return new MethodTarget(found.iterator().next(), bean, help, availabilityIndicator);
 	}
 
 	public Method getMethod() {
@@ -72,6 +91,10 @@ public class MethodTarget {
 		return help;
 	}
 
+	public Availability getAvailability() {
+		return availabilityIndicator.get();
+	}
+
 	@Override
 	public boolean equals(Object o) {
 		if (this == o) return true;

spring-shell-core/src/main/java/org/springframework/shell/Shell.java

@@ -131,16 +131,21 @@ public class Shell implements CommandRegistry {
 		Object result;
 		if (command != null) {
 			MethodTarget methodTarget = methodTargets.get(command);
-			List<String> wordsForArgs = wordsForArguments(command, words);
-			Method method = methodTarget.getMethod();
+			Availability availability = methodTarget.getAvailability();
+			if (availability.isAvailable()) {
+				List<String> wordsForArgs = wordsForArguments(command, words);
+				Method method = methodTarget.getMethod();
 
-			try {
-				Object[] args = resolveArgs(method, wordsForArgs);
-				validateArgs(args, methodTarget);
-				result = ReflectionUtils.invokeMethod(method, methodTarget.getBean(), args);
-			}
-			catch (Exception e) {
-				result = e;
+				try {
+					Object[] args = resolveArgs(method, wordsForArgs);
+					validateArgs(args, methodTarget);
+					result = ReflectionUtils.invokeMethod(method, methodTarget.getBean(), args);
+				}
+				catch (Exception e) {
+					result = e;
+				}
+			} else {
+				result = new CommandNotCurrentlyAvailable(command, availability);
 			}
 		}
 		else {