Improve variables view

We used to have two scopes of variables, "locals" and "arguments".
However, VM somehow returns incorrect results for
LocalVariable#isArgument(), so we presented confusing views to users.
Since it's not very important to distinguish arguments and local
variables, we now include them in the same "locals" scope.

Instead, we introduce the new "fields" scope containing fields of the
current object (aka "this").

And finally, we now support inspecting the hierarchy of objects.

Author: nya3jp

src/main/java/org/javacs/debug/JavaDebugServer.java

@@ -13,6 +13,7 @@
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
@@ -440,6 +441,7 @@ public void terminate(TerminateArguments req) {
 
     @Override
     public void continue_(ContinueArguments req) {
+        valueIdTracker.clear();
         vm.resume();
     }
 
@@ -454,6 +456,7 @@ public void next(NextArguments req) {
         var step = vm.eventRequestManager().createStepRequest(thread, StepRequest.STEP_LINE, StepRequest.STEP_OVER);
         step.addCountFilter(1);
         step.enable();
+        valueIdTracker.clear();
         vm.resume();
     }
 
@@ -468,6 +471,7 @@ public void stepIn(StepInArguments req) {
         var step = vm.eventRequestManager().createStepRequest(thread, StepRequest.STEP_LINE, StepRequest.STEP_INTO);
         step.addCountFilter(1);
         step.enable();
+        valueIdTracker.clear();
         vm.resume();
     }
 
@@ -482,6 +486,7 @@ public void stepOut(StepOutArguments req) {
         var step = vm.eventRequestManager().createStepRequest(thread, StepRequest.STEP_LINE, StepRequest.STEP_OUT);
         step.addCountFilter(1);
         step.enable();
+        valueIdTracker.clear();
         vm.resume();
     }
 
@@ -632,47 +637,157 @@ private com.sun.jdi.StackFrame findFrame(long id) {
     @Override
     public ScopesResponseBody scopes(ScopesArguments req) {
         var resp = new ScopesResponseBody();
+        var fields = new Scope();
+        fields.name = "Fields";
+        fields.presentationHint = "locals";
+        fields.expensive = true; // do not expand by default
+        fields.variablesReference = req.frameId * 2;
         var locals = new Scope();
         locals.name = "Locals";
         locals.presentationHint = "locals";
-        locals.variablesReference = req.frameId * 2;
-        var arguments = new Scope();
-        arguments.name = "Arguments";
-        arguments.presentationHint = "arguments";
-        arguments.variablesReference = req.frameId * 2 + 1;
-        resp.scopes = new Scope[] {locals, arguments};
+        locals.variablesReference = req.frameId * 2 + 1;
+        resp.scopes = new Scope[] {fields, locals};
         return resp;
     }
 
+    private static final long VALUE_ID_START = 1000000000;
+
+    private static class ValueIdTracker {
+        private final HashMap<Long, Value> values = new HashMap<>();
+        private long nextId = VALUE_ID_START;
+
+        public void clear() {
+            values.clear();
+            // Keep nextId to avoid accidentally accessing wrong Values.
+        }
+
+        public Value get(long id) {
+            return values.get(id);
+        }
+
+        public long put(Value value) {
+            long id = nextId++;
+            values.put(id, value);
+            return id;
+        }
+    }
+
+    private final ValueIdTracker valueIdTracker = new ValueIdTracker();
+
+    private static boolean hasInterestingChildren(Value value) {
+        return value instanceof ObjectReference && !(value instanceof StringReference);
+    }
+
     @Override
     public VariablesResponseBody variables(VariablesArguments req) {
-        var frameId = req.variablesReference / 2;
-        var scopeId = (int) (req.variablesReference % 2);
-        var argumentScope = scopeId == 1;
+        if (req.variablesReference < VALUE_ID_START) {
+            var frameId = req.variablesReference / 2;
+            var scopeId = (int)(req.variablesReference % 2);
+            return frameVariables(frameId, scopeId);
+        }
+        Value value = valueIdTracker.get(req.variablesReference);
+        return valueChildren(value);
+    }
+
+    private VariablesResponseBody frameVariables(long frameId, int scopeId) {
         var frame = findFrame(frameId);
+        var thread = frame.thread();
+        var variables = new ArrayList<Variable>();
+
+        if (scopeId == 0) {
+            var thisValue = frame.thisObject();
+            if (thisValue != null) {
+                variables.addAll(objectFieldsAsVariables(thisValue, thread));
+            }
+        } else {
+            variables.addAll(frameLocalsAsVariables(frame, thread));
+        }
+
+        var resp = new VariablesResponseBody();
+        resp.variables = variables.toArray(Variable[]::new);
+        return resp;
+    }
+
+    private VariablesResponseBody valueChildren(Value parentValue) {
+        // TODO: Use an actual owner thread.
+        ThreadReference mainThread = vm.allThreads().get(0);
+        var variables = new ArrayList<Variable>();
+
+        if (parentValue instanceof ArrayReference array) {
+            variables.addAll(arrayElementsAsVariables(array, mainThread));
+        } else if (parentValue instanceof ObjectReference object) {
+            variables.addAll(objectFieldsAsVariables(object, mainThread));
+        }
+
+        var resp = new VariablesResponseBody();
+        resp.variables = variables.toArray(Variable[]::new);
+        return resp;
+    }
+
+    private List<Variable> frameLocalsAsVariables(com.sun.jdi.StackFrame frame, ThreadReference thread) {
         List<LocalVariable> visible;
         try {
             visible = frame.visibleVariables();
         } catch (AbsentInformationException __) {
             LOG.warning(String.format("No visible variable information in %s", frame.location()));
-            return new VariablesResponseBody();
+            return List.of();
         }
-        var values = frame.getValues(visible);
-        var thread = frame.thread();
+
         var variables = new ArrayList<Variable>();
+        var values = frame.getValues(visible);
         for (var v : visible) {
-            if (v.isArgument() != argumentScope) continue;
+            var value = values.get(v);
             var w = new Variable();
             w.name = v.name();
-            w.value = print(values.get(v), thread);
+            w.value = print(value, thread);
             w.type = v.typeName();
-            // TODO set variablesReference and allow inspecting structure of collections and POJOs
+            if (hasInterestingChildren(value)) {
+                w.variablesReference = valueIdTracker.put(value);
+            }
+            if (value instanceof ArrayReference array) {
+                w.indexedVariables = array.length();
+            }
             // TODO set variablePresentationHint
             variables.add(w);
         }
-        var resp = new VariablesResponseBody();
-        resp.variables = variables.toArray(Variable[]::new);
-        return resp;
+        return variables;
+    }
+
+    private List<Variable> arrayElementsAsVariables(ArrayReference array, ThreadReference thread) {
+        var variables = new ArrayList<Variable>();
+        var arrayType = (ArrayType) array.type();
+        var values = array.getValues();
+        var length = values.size();
+        for (int i = 0; i < length; i++) {
+            var value = values.get(i);
+            var w = new Variable();
+            w.name = Integer.toString(i, 10);
+            w.value = print(value, thread);
+            w.type = arrayType.componentTypeName();
+            if (hasInterestingChildren(value)) {
+                w.variablesReference = valueIdTracker.put(value);
+            }
+            variables.add(w);
+        }
+        return variables;
+    }
+
+    private List<Variable> objectFieldsAsVariables(ObjectReference object, ThreadReference thread) {
+        var variables = new ArrayList<Variable>();
+        var classType = (ClassType) object.type();
+        var values = object.getValues(classType.allFields());
+        for (var field : values.keySet()) {
+            var value = values.get(field);
+            var w = new Variable();
+            w.name = field.name();
+            w.value = print(value, thread);
+            w.type = field.typeName();
+            if (hasInterestingChildren(value)) {
+                w.variablesReference = valueIdTracker.put(value);
+            }
+            variables.add(w);
+        }
+        return variables;
     }
 
     private String print(Value value, ThreadReference t) {