diff --git a/spring-shell2-core/src/main/java/org/springframework/shell2/AbstractShell.java b/spring-shell2-core/src/main/java/org/springframework/shell2/AbstractShell.java
new file mode 100644
index 00000000..becf0e11
--- /dev/null
+++ b/spring-shell2-core/src/main/java/org/springframework/shell2/AbstractShell.java
@@ -0,0 +1,234 @@
+/*
+ * 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.shell2;
+
+import java.io.IOException;
+import java.lang.reflect.Method;
+import java.lang.reflect.Parameter;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import javax.annotation.PostConstruct;
+import javax.validation.ConstraintViolation;
+import javax.validation.Validation;
+import javax.validation.executable.ExecutableValidator;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.context.ApplicationContext;
+import org.springframework.core.MethodParameter;
+import org.springframework.util.ReflectionUtils;
+
+/**
+ * Base class implementing a shell loop.
+ *
+ * <p>Given some textual input, locate the {@link MethodTarget} to invoke and {@link ResultHandler#handleResult(Object) handle}
+ * the result.</p>
+ *
+ * <p>Also provides hooks for code completion</p>
+ *
+ * @author Eric Bottard
+ */
+public abstract class AbstractShell implements Shell {
+
+	@Autowired
+	@Qualifier("main")
+	ResultHandler resultHandler;
+
+	@Autowired
+	protected ApplicationContext applicationContext;
+
+	protected Map<String, MethodTarget> methodTargets = new HashMap<>();
+
+	@Autowired
+	protected List<ParameterResolver> parameterResolvers = new ArrayList<>();
+
+	/**
+	 * Marker object to distinguish unresolved arguments from {@code null}, which is a valid value.
+	 */
+	protected static final Object UNRESOLVED = new Object();
+
+	@Override
+	public Map<String, MethodTarget> listCommands() {
+		return methodTargets;
+	}
+
+	@PostConstruct
+	public void gatherMethodTargets() throws Exception {
+		for (MethodTargetResolver resolver : applicationContext.getBeansOfType(MethodTargetResolver.class).values()) {
+			methodTargets.putAll(resolver.resolve());
+		}
+	}
+
+	/**
+	 * The main program loop: acquire input, try to match it to a command and evaluate. Repeat until a
+	 * {@link ResultHandler} causes the process to exit.
+	 */
+	public void run() throws IOException {
+		while (true) {
+			Input input = readInput();
+			if (input.words().isEmpty()) {
+				continue;
+			}
+
+
+			String line = input.rawText();
+			List<String> words = input.words();
+
+			String command = findLongestCommand(line);
+
+			if (command != null) {
+				int wordsUsedForCommandKey = command.split(" ").length;
+				MethodTarget methodTarget = methodTargets.get(command);
+				List<String> wordsForArgs = words.subList(wordsUsedForCommandKey, words.size());
+				Method method = methodTarget.getMethod();
+
+				Object result = null;
+				try {
+					Object[] args = resolveArgs(method, wordsForArgs);
+					validateArgs(args, methodTarget);
+					result = ReflectionUtils.invokeMethod(method, methodTarget.getBean(), args);
+				}
+				catch (Exception e) {
+					result = e;
+				}
+
+				resultHandler.handleResult(result);
+
+			}
+			else {
+				System.out.println("No command found for " + words);
+			}
+		}
+	}
+
+	/**
+	 * Return text entered by user to invoke commands.
+	 */
+	protected abstract Input readInput();
+
+	/**
+	 * Gather completion proposals given some (incomplete) input the user has already typed in.
+	 * When and how this method is invoked is implementation specific and decided by subclasses.
+	 */
+	public List<CompletionProposal> complete(CompletionContext context) {
+
+		String prefix = context.upToCursor();
+
+		List<CompletionProposal> candidates = new ArrayList<>();
+		// Find the longest match for a command name with words in the buffer
+		String best = findLongestCommand(prefix);
+		if (best == null) { // no command found
+			candidates.addAll(commandsStartingWith(prefix));
+			return candidates;
+		} // if we're here, we're either trying to complete args for command <best> (will fall through)
+		// or trying to complete command whose name starts with <best> (which also happens to be a command)
+		else if (prefix.equals(best)) {
+			candidates.addAll(commandsStartingWith(best));
+		} // valid command (<best>) followed by a suffix (but not necessarily [<space> args*])
+		else if (!prefix.startsWith(best + " ")) {
+			// must be an invalid command, can't do anything
+			return candidates;
+		}
+
+		// Try to complete arguments
+		MethodTarget methodTarget = methodTargets.get(best);
+		Method method = methodTarget.getMethod();
+		return Arrays.stream(method.getParameters())
+			.map(Utils::createMethodParameter)
+			.flatMap(mp -> findResolver(mp).complete(mp, context).stream())
+			.collect(Collectors.toList());
+	}
+
+	private List<CompletionProposal> commandsStartingWith(String prefix) {
+		return methodTargets.entrySet().stream()
+			.filter(e -> e.getKey().startsWith(prefix))
+			.map(e -> toCompletionProposal(e.getKey(), e.getValue()))
+			.collect(Collectors.toList());
+	}
+
+	private CompletionProposal toCompletionProposal(String command, MethodTarget methodTarget) {
+		return new CompletionProposal(command)
+			.category("Available commands")
+			.description(methodTarget.getHelp());
+	}
+
+	private void validateArgs(Object[] args, MethodTarget methodTarget) {
+		for (int i = 0; i < args.length; i++) {
+			if (args[i] == UNRESOLVED) {
+				MethodParameter methodParameter = Utils.createMethodParameter(methodTarget.getMethod(), i);
+				throw new IllegalStateException("Could not resolve " + methodParameter);
+			}
+		}
+		ExecutableValidator executableValidator = Validation
+			.buildDefaultValidatorFactory().getValidator().forExecutables();
+		Set<ConstraintViolation<Object>> constraintViolations = executableValidator.validateParameters(methodTarget.getBean(),
+			methodTarget.getMethod(),
+			args);
+		if (constraintViolations.size() > 0) {
+			System.out.println(constraintViolations);
+		}
+	}
+
+	/**
+	 * Use all known {@link ParameterResolver}s to try to compute a value for each parameter of the method to
+	 * invoke.
+	 * @param method       the method for which parameters should be computed
+	 * @param wordsForArgs the list of 'words' that should be converted to parameter values.
+	 *                     May include markers for passing parameters 'by name'
+	 * @return an array containing resolved parameter values, or {@link #UNRESOLVED} for parameters that could not be
+	 * resolved
+	 */
+	private Object[] resolveArgs(Method method, List<String> wordsForArgs) {
+		Parameter[] parameters = method.getParameters();
+		Object[] args = new Object[parameters.length];
+		Arrays.fill(args, UNRESOLVED);
+		for (int i = 0; i < parameters.length; i++) {
+			MethodParameter methodParameter = Utils.createMethodParameter(method, i);
+			args[i] = findResolver(methodParameter).resolve(methodParameter, wordsForArgs);
+		}
+		return args;
+	}
+
+	protected ParameterResolver findResolver(MethodParameter parameter) {
+		return parameterResolvers.stream()
+			.filter(resolver -> resolver.supports(parameter))
+			.findFirst()
+			.orElseThrow(() -> new RuntimeException("resolver not found"));
+	}
+
+
+	/**
+	 * Returns the longest command that can be matched as first word(s) in the given buffer.
+	 *
+	 * @return a valid command name, or {@literal null} if none matched
+	 */
+	protected String findLongestCommand(String prefix) {
+		String result = methodTargets.keySet().stream()
+			.filter(prefix::startsWith)
+			.reduce("", (c1, c2) -> c1.length() > c2.length() ? c1 : c2);
+		return "".equals(result) ? null : result;
+	}
+
+
+
+}
diff --git a/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionContext.java b/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionContext.java
index 1854c899..6a94683b 100644
--- a/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionContext.java
+++ b/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionContext.java
@@ -59,7 +59,10 @@ public class CompletionContext {
 	public String upToCursor() {
 		String start = words.subList(0, wordIndex).stream().collect(Collectors.joining(" "));
 		if (wordIndex < words.size()) {
-			start += " " + currentWord().substring(0, position);
+			if (!start.isEmpty()) {
+				start += " ";
+			}
+			start += currentWord().substring(0, position);
 		}
 		return start;
 	}
diff --git a/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionProposal.java b/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionProposal.java
index 14e0a9e7..d1ef5c04 100644
--- a/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionProposal.java
+++ b/spring-shell2-core/src/main/java/org/springframework/shell2/CompletionProposal.java
@@ -51,32 +51,36 @@ public class CompletionProposal {
 		return value;
 	}
 
-	public void value(String value) {
+	public CompletionProposal value(String value) {
 		this.value = value;
+		return this;
 	}
 
 	public String displayText() {
 		return displayText;
 	}
 
-	public void displayText(String displayText) {
+	public CompletionProposal displayText(String displayText) {
 		this.displayText = displayText;
+		return this;
 	}
 
 	public String description() {
 		return description;
 	}
 
-	public void description(String description) {
+	public CompletionProposal description(String description) {
 		this.description = description;
+		return this;
 	}
 
 	public String category() {
 		return category;
 	}
 
-	public void category(String category) {
+	public CompletionProposal category(String category) {
 		this.category = category;
+		return this;
 	}
 
 	@Override
diff --git a/spring-shell2-core/src/main/java/org/springframework/shell2/Input.java b/spring-shell2-core/src/main/java/org/springframework/shell2/Input.java
new file mode 100644
index 00000000..6dbc0ed7
--- /dev/null
+++ b/spring-shell2-core/src/main/java/org/springframework/shell2/Input.java
@@ -0,0 +1,52 @@
+/*
+ * 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.shell2;
+
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Represents the input buffer to the shell.
+ *
+ * @author Eric Bottard
+ */
+public interface Input {
+
+	Input EMPTY = new Input() {
+		@Override
+		public String rawText() {
+			return "";
+		}
+
+		@Override
+		public List<String> words() {
+			return Collections.emptyList();
+		}
+	};
+
+	/**
+	 * Return the input as entered by the user.
+	 */
+	String rawText();
+
+	/**
+	 * Return the input as a list of parsed "words", having split the raw input according
+	 * to parsing rules (for example, handling quoted portions of the readInput as a single
+	 * "word")
+	 */
+	List<String> words();
+}
diff --git a/spring-shell2-core/src/main/java/org/springframework/shell2/JLineShell.java b/spring-shell2-core/src/main/java/org/springframework/shell2/JLineShell.java
index 5649c4d1..9e2398b0 100644
--- a/spring-shell2-core/src/main/java/org/springframework/shell2/JLineShell.java
+++ b/spring-shell2-core/src/main/java/org/springframework/shell2/JLineShell.java
@@ -16,22 +16,10 @@
 
 package org.springframework.shell2;
 
-import java.io.IOException;
-import java.lang.reflect.Method;
-import java.lang.reflect.Parameter;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
-import java.util.Set;
-import java.util.function.Function;
 import java.util.stream.Collectors;
 
 import javax.annotation.PostConstruct;
-import javax.validation.ConstraintViolation;
-import javax.validation.Validation;
-import javax.validation.executable.ExecutableValidator;
 
 import org.jline.reader.Candidate;
 import org.jline.reader.Completer;
@@ -40,63 +28,30 @@ import org.jline.reader.LineReader;
 import org.jline.reader.LineReaderBuilder;
 import org.jline.reader.ParsedLine;
 import org.jline.reader.UserInterruptException;
-import org.jline.reader.impl.DefaultParser;
 import org.jline.terminal.Terminal;
 import org.jline.utils.AttributedString;
 import org.jline.utils.AttributedStringBuilder;
 import org.jline.utils.AttributedStyle;
+
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.beans.factory.annotation.Qualifier;
-import org.springframework.context.ApplicationContext;
-import org.springframework.core.MethodParameter;
 import org.springframework.stereotype.Component;
-import org.springframework.util.ReflectionUtils;
 
 /**
- * Main component implementing a REPL using JLine.
- *
- * <p>Discovers {@link MethodTarget}s at startup and hands off execution of commands according
- * to the parsed JLine buffer.</p>
+ * Shell implementation using JLine to capture input and trigger completions.
  *
  * @author Eric Bottard
  * @author Florent Biville
  */
 @Component
-public class JLineShell implements Shell {
-
-	@Autowired
-	@Qualifier("main")
-	ResultHandler resultHandler;
-
-	@Autowired
-	private ApplicationContext applicationContext;
-
-	private Map<String, MethodTarget> methodTargets = new HashMap<>();
+public class JLineShell extends AbstractShell {
 
 	LineReader lineReader;
 
 	@Autowired
 	private Terminal terminal;
 
-	@Autowired
-	private List<ParameterResolver> parameterResolvers = new ArrayList<>();
-
-	/**
-	 * Marker object to distinguish unresolved arguments from {@code null}, which is a valid value.
-	 */
-	private static final Object UNRESOLVED = new Object();
-
-	@Override
-	public Map<String, MethodTarget> listCommands() {
-		return methodTargets;
-	}
-
 	@PostConstruct
 	public void init() throws Exception {
-		for (MethodTargetResolver resolver : applicationContext.getBeansOfType(MethodTargetResolver.class).values()) {
-			methodTargets.putAll(resolver.resolve());
-		}
-
 		ExtendedDefaultParser parser = new ExtendedDefaultParser();
 		parser.setEofOnUnclosedQuote(true);
 		parser.setEofOnEscapedNewLine(true);
@@ -132,53 +87,26 @@ public class JLineShell implements Shell {
 	}
 
 
-	public void run() throws IOException {
-		while (true) {
-			try {
-				lineReader.readLine(new AttributedString("shell:>", AttributedStyle.DEFAULT.foreground(AttributedStyle.YELLOW)).toAnsi(terminal));
-			}
-			catch (UserInterruptException e) {
-				if (e.getPartialLine().isEmpty()) {
-					resultHandler.handleResult(new ExitRequest(1));
-				} else {
-					continue;
-				}
-			}
-
-			String line = lineReader.getParsedLine().line();
-			String command = findLongestCommand(line);
-
-			List<String> words = lineReader.getParsedLine().words();
-			if (command != null) {
-				int wordsUsedForCommandKey = command.split(" ").length;
-				MethodTarget methodTarget = methodTargets.get(command);
-				List<String> wordsForArgs = sanitizeInput(words.subList(wordsUsedForCommandKey, words.size()));
-				Method method = methodTarget.getMethod();
-
-				Object result = null;
-				try {
-					Object[] args = resolveArgs(method, wordsForArgs);
-					validateArgs(args, methodTarget);
-					result = ReflectionUtils.invokeMethod(method, methodTarget.getBean(), args);
-				}
-				catch (Exception e) {
-					result = e;
-				}
-
-				resultHandler.handleResult(result);
-
-			}
-			else {
-				System.out.println("No command found for " + sanitizeInput(words));
+	@Override
+	protected Input readInput() {
+		try {
+			lineReader.readLine(new AttributedString("shell:>", AttributedStyle.DEFAULT.foreground(AttributedStyle.YELLOW)).toAnsi(terminal));
+		}
+		catch (UserInterruptException e) {
+			if (e.getPartialLine().isEmpty()) {
+				resultHandler.handleResult(new ExitRequest(1));
+			} else {
+				return Input.EMPTY;
 			}
 		}
+		return new JLineInput(lineReader.getParsedLine());
 	}
 
 	/**
 	 * Sanitize the buffer input given the customizations applied to the JLine parser (<em>e.g.</em> support for
 	 * line continuations, <em>etc.</em>)
 	 */
-	private List<String> sanitizeInput(List<String> words) {
+	static private List<String> sanitizeInput(List<String> words) {
 		words = words.stream()
 			.map(s -> s.replaceAll("^\\n+|\\n+$", "")) // CR at beginning/end of line introduced by backslash continuation
 			.map(s -> s.replaceAll("\\n+", " ")) // CR in middle of word introduced by return inside a quoted string
@@ -187,48 +115,12 @@ public class JLineShell implements Shell {
 		return words;
 	}
 
-	private void validateArgs(Object[] args, MethodTarget methodTarget) {
-		for (int i = 0; i < args.length; i++) {
-			if (args[i] == UNRESOLVED) {
-				MethodParameter methodParameter = Utils.createMethodParameter(methodTarget.getMethod(), i);
-				throw new IllegalStateException("Could not resolve " + methodParameter);
-			}
-		}
-		ExecutableValidator executableValidator = Validation
-				.buildDefaultValidatorFactory().getValidator().forExecutables();
-		Set<ConstraintViolation<Object>> constraintViolations = executableValidator.validateParameters(methodTarget.getBean(),
-				methodTarget.getMethod(),
-				args);
-		if (constraintViolations.size() > 0) {
-			System.out.println(constraintViolations);
-		}
-	}
+	// Overridden so it can be called from CompleterAdapter
 
-	/**
-	 * Use all known {@link ParameterResolver}s to try to compute a value for each parameter of the method to
-	 * invoke.
-	 * @param method       the method for which parameters should be computed
-	 * @param wordsForArgs the list of 'words' that should be converted to parameter values.
-	 *                     May include markers for passing parameters 'by name'
-	 * @return an array containing resolved parameter values, or {@link #UNRESOLVED} for parameters that could not be
-	 * resolved
-	 */
-	private Object[] resolveArgs(Method method, List<String> wordsForArgs) {
-		Parameter[] parameters = method.getParameters();
-		Object[] args = new Object[parameters.length];
-		Arrays.fill(args, UNRESOLVED);
-		for (int i = 0; i < parameters.length; i++) {
-			MethodParameter methodParameter = Utils.createMethodParameter(method, i);
-			args[i] = findResolver(methodParameter).resolve(methodParameter, wordsForArgs);
-		}
-		return args;
-	}
 
-	private ParameterResolver findResolver(MethodParameter parameter) {
-		return parameterResolvers.stream()
-				.filter(resolver -> resolver.supports(parameter))
-				.findFirst()
-				.orElseThrow(() -> new RuntimeException("resolver not found"));
+	@Override
+	public List<CompletionProposal> complete(CompletionContext context) {
+		return super.complete(context);
 	}
 
 	/**
@@ -239,72 +131,43 @@ public class JLineShell implements Shell {
 
 		@Override
 		public void complete(LineReader reader, ParsedLine line, List<Candidate> candidates) {
-			String prefix = reader.getBuffer().upToCursor();
-
-			// Find the longest match for a command name with words in the buffer
-			String best = findLongestCommand(prefix);
-			if (best == null) { // no command found
-				candidates.addAll(commandsStartingWith(prefix));
-				return;
-			} // if we're here, we're either trying to complete args for command <best> (will fall through)
-			// or trying to complete command whose name starts with <best> (which also happens to be a command)
-			else if (prefix.equals(best)) {
-				candidates.addAll(commandsStartingWith(best));
-			} // valid command (<best>) followed by a suffix (but not necessarily [<space> args*])
-			else if (!prefix.startsWith(best + " ")) {
-				// must be an invalid command, can't do anything
-				return;
-			}
-
 			CompletingParsedLine cpl = (line instanceof CompletingParsedLine) ? ((CompletingParsedLine) line) : t -> t;
 
-			// Try to complete arguments
-			MethodTarget methodTarget = methodTargets.get(best);
-			List<String> words = line.words();
-			int noOfWordsInCommand = best.split(" ").length;
-			List<String> rest = words.subList(noOfWordsInCommand, words.size())
-					.stream()
-					.filter(w -> !w.isEmpty())
-					.collect(Collectors.toList());
-			CompletionContext context = new CompletionContext(rest, line.wordIndex() - noOfWordsInCommand, line.wordCursor());
-			Method method = methodTarget.getMethod();
-			for (int i = 0; i < method.getParameterCount(); i++) {
-				MethodParameter methodParameter = Utils.createMethodParameter(method, i);
-				ParameterResolver resolver = findResolver(methodParameter);
-				resolver.complete(methodParameter, context)
-						.stream()
-						.map(completion -> new Candidate(
-										cpl.emit(completion.value()).toString(),
-										completion.displayText(),
-										"Value for parameter " + resolver.describe(methodParameter).toString(),
-										resolver.describe(methodParameter).help(),
-										null, null, true)
-						)
-						.forEach(candidates::add);
-			}
-		}
+			CompletionContext context = new CompletionContext(sanitizeInput(line.words()), line.wordIndex(), line.wordCursor());
 
-		private List<Candidate> commandsStartingWith(String prefix) {
-			return methodTargets.entrySet().stream()
-					.filter(e -> e.getKey().startsWith(prefix)) // find commands that start with our buffer prefix
-					.map(e -> toCandidate(e.getKey(), e.getValue()))
-					.collect(Collectors.toList());
-		}
-
-		private Candidate toCandidate(String command, MethodTarget methodTarget) {
-			return new Candidate(command, command, "Available commands", methodTarget.getHelp(), null, null, true);
+			List<CompletionProposal> proposals = JLineShell.this.complete(context);
+			proposals.stream()
+				.map(p -> new Candidate(
+					cpl.emit(p.value()).toString(),
+					p.displayText(),
+					p.category(),
+					p.description(),
+					null,
+					null,
+					true)
+				)
+				.forEach(candidates::add);
 		}
 	}
 
-	/**
-	 * Returns the longest command that can be matched as first word(s) in the given buffer.
-	 *
-	 * @return a valid command name, or {@literal null} if none matched
-	 */
-	private String findLongestCommand(String prefix) {
-		String result = methodTargets.keySet().stream()
-			.filter(prefix::startsWith)
-			.reduce("", (c1, c2) -> c1.length() > c2.length() ? c1 : c2);
-		return "".equals(result) ? null : result;
+	private static class JLineInput implements Input {
+
+		private final ParsedLine parsedLine;
+
+		JLineInput(ParsedLine parsedLine) {
+			this.parsedLine = parsedLine;
+		}
+
+		@Override
+		public String rawText() {
+			return parsedLine.line();
+		}
+
+		@Override
+		public List<String> words() {
+			return sanitizeInput(parsedLine.words());
+		}
 	}
+
 }
+