diff --git a/src/pkg/json/generic.go b/src/pkg/json/generic.go
index 7486db5679b..d53c4f6b0bd 100644
--- a/src/pkg/json/generic.go
+++ b/src/pkg/json/generic.go
@@ -364,6 +364,9 @@ func (b *_JsonBuilder) Key(k string) Builder {
 	return bb;
 }
 
+func (b *_JsonBuilder) Flush() {
+}
+
 // StringToJson parses the string s as a JSON-syntax string
 // and returns the generic JSON object representation.
 // On success, StringToJson returns with ok set to true and errtok empty.
diff --git a/src/pkg/json/parse.go b/src/pkg/json/parse.go
index af610b5d1b7..ba291e940a1 100644
--- a/src/pkg/json/parse.go
+++ b/src/pkg/json/parse.go
@@ -296,6 +296,9 @@ type Builder interface {
 	// Create sub-Builders
 	Elem(i int) Builder;
 	Key(s string) Builder;
+
+	// Flush changes to parent Builder if necessary.
+	Flush();
 }
 
 func parse(lex *_Lexer, build Builder) bool {
@@ -392,6 +395,7 @@ Switch:
 	if ok {
 		lex.Next();
 	}
+	build.Flush();
 	return ok;
 }
 
diff --git a/src/pkg/json/struct.go b/src/pkg/json/struct.go
index 4fc70a814bb..03e2966c888 100644
--- a/src/pkg/json/struct.go
+++ b/src/pkg/json/struct.go
@@ -12,11 +12,15 @@ import (
 	"strings";
 )
 
-type _StructBuilder struct {
+type structBuilder struct {
 	val reflect.Value;
+
+	// if map_ != nil, write val to map_[key] on each change
+	map_ *reflect.MapValue;
+	key reflect.Value;
 }
 
-var nobuilder *_StructBuilder
+var nobuilder *structBuilder
 
 func isfloat(v reflect.Value) bool {
 	switch v.(type) {
@@ -62,7 +66,18 @@ func setint(v reflect.Value, i int64) {
 	}
 }
 
-func (b *_StructBuilder) Int64(i int64) {
+// If updating b.val is not enough to update the original,
+// copy a changed b.val out to the original.
+func (b *structBuilder) Flush() {
+	if b == nil {
+		return;
+	}
+	if b.map_ != nil {
+		b.map_.SetElem(b.key, b.val);
+	}
+}
+
+func (b *structBuilder) Int64(i int64) {
 	if b == nil {
 		return;
 	}
@@ -74,7 +89,7 @@ func (b *_StructBuilder) Int64(i int64) {
 	}
 }
 
-func (b *_StructBuilder) Uint64(i uint64) {
+func (b *structBuilder) Uint64(i uint64) {
 	if b == nil {
 		return;
 	}
@@ -86,7 +101,7 @@ func (b *_StructBuilder) Uint64(i uint64) {
 	}
 }
 
-func (b *_StructBuilder) Float64(f float64) {
+func (b *structBuilder) Float64(f float64) {
 	if b == nil {
 		return;
 	}
@@ -98,9 +113,9 @@ func (b *_StructBuilder) Float64(f float64) {
 	}
 }
 
-func (b *_StructBuilder) Null() {}
+func (b *structBuilder) Null() {}
 
-func (b *_StructBuilder) String(s string) {
+func (b *structBuilder) String(s string) {
 	if b == nil {
 		return;
 	}
@@ -109,7 +124,7 @@ func (b *_StructBuilder) String(s string) {
 	}
 }
 
-func (b *_StructBuilder) Bool(tf bool) {
+func (b *structBuilder) Bool(tf bool) {
 	if b == nil {
 		return;
 	}
@@ -118,7 +133,7 @@ func (b *_StructBuilder) Bool(tf bool) {
 	}
 }
 
-func (b *_StructBuilder) Array() {
+func (b *structBuilder) Array() {
 	if b == nil {
 		return;
 	}
@@ -129,14 +144,14 @@ func (b *_StructBuilder) Array() {
 	}
 }
 
-func (b *_StructBuilder) Elem(i int) Builder {
+func (b *structBuilder) Elem(i int) Builder {
 	if b == nil || i < 0 {
 		return nobuilder;
 	}
 	switch v := b.val.(type) {
 	case *reflect.ArrayValue:
 		if i < v.Len() {
-			return &_StructBuilder{v.Elem(i)};
+			return &structBuilder{val: v.Elem(i)};
 		}
 	case *reflect.SliceValue:
 		if i > v.Cap() {
@@ -155,36 +170,55 @@ func (b *_StructBuilder) Elem(i int) Builder {
 			v.SetLen(i+1);
 		}
 		if i < v.Len() {
-			return &_StructBuilder{v.Elem(i)};
+			return &structBuilder{val: v.Elem(i)};
 		}
 	}
 	return nobuilder;
 }
 
-func (b *_StructBuilder) Map() {
+func (b *structBuilder) Map() {
 	if b == nil {
 		return;
 	}
-	if v, ok := b.val.(*reflect.PtrValue); ok {
+	if v, ok := b.val.(*reflect.PtrValue); ok && v.IsNil() {
 		if v.IsNil() {
 			v.PointTo(reflect.MakeZero(v.Type().(*reflect.PtrType).Elem()));
+			b.Flush();
 		}
+		b.map_ = nil;
+		b.val = v.Elem();
+	}
+	if v, ok := b.val.(*reflect.MapValue); ok && v.IsNil() {
+		v.Set(reflect.MakeMap(v.Type().(*reflect.MapType)));
 	}
 }
 
-func (b *_StructBuilder) Key(k string) Builder {
+func (b *structBuilder) Key(k string) Builder {
 	if b == nil {
 		return nobuilder;
 	}
-	if v, ok := reflect.Indirect(b.val).(*reflect.StructValue); ok {
+	switch v := reflect.Indirect(b.val).(type) {
+	case *reflect.StructValue:
 		t := v.Type().(*reflect.StructType);
 		// Case-insensitive field lookup.
 		k = strings.ToLower(k);
 		for i := 0; i < t.NumField(); i++ {
 			if strings.ToLower(t.Field(i).Name) == k {
-				return &_StructBuilder{v.Field(i)};
+				return &structBuilder{val: v.Field(i)};
 			}
 		}
+	case *reflect.MapValue:
+		t := v.Type().(*reflect.MapType);
+		if t.Key() != reflect.Typeof(k) {
+			break;
+		}
+		key := reflect.NewValue(k);
+		elem := v.Elem(key);
+		if elem == nil {
+			v.SetElem(key, reflect.MakeZero(t.Elem()));
+			elem = v.Elem(key);
+		}
+		return &structBuilder{val: elem, map_: v, key: key};
 	}
 	return nobuilder;
 }
@@ -249,7 +283,7 @@ func (b *_StructBuilder) Key(k string) Builder {
 // On a syntax error, it returns with ok set to false and errtok
 // set to the offending token.
 func Unmarshal(s string, val interface{}) (ok bool, errtok string) {
-	b := &_StructBuilder{reflect.NewValue(val)};
+	b := &structBuilder{val: reflect.NewValue(val)};
 	ok, _, errtok = Parse(s, b);
 	if !ok {
 		return false, errtok;
diff --git a/src/pkg/json/struct_test.go b/src/pkg/json/struct_test.go
index 42b54b9dbb8..bdcc2fdddf1 100644
--- a/src/pkg/json/struct_test.go
+++ b/src/pkg/json/struct_test.go
@@ -5,10 +5,11 @@
 package json
 
 import (
+	"reflect";
 	"testing";
 )
 
-type _MyStruct struct {
+type myStruct struct {
 	T	bool;
 	F	bool;
 	S	string;
@@ -26,17 +27,36 @@ type _MyStruct struct {
 	Fl32	float32;
 	Fl64	float64;
 	A	[]string;
-	My	*_MyStruct;
+	My	*myStruct;
+	Map	map[string][]int;
+	MapStruct map[string]myStruct;
+	MapPtrStruct map[string]*myStruct;
 }
 
-const _Encoded = `{"t":true,"f":false,"s":"abc","i8":1,"i16":2,"i32":3,"i64":4,`
+const encoded = `{"t":true,"f":false,"s":"abc","i8":1,"i16":2,"i32":3,"i64":4,`
 	` "u8":5,"u16":6,"u32":7,"u64":8,`
 	` "i":-9,"u":10,"bogusfield":"should be ignored",`
 	` "fl":11.5,"fl32":12.25,"fl64":13.75,`
-	` "a":["x","y","z"],"my":{"s":"subguy"}}`
+	` "a":["x","y","z"],"my":{"s":"subguy"},`
+	`"map":{"k1":[1,2,3],"k2":[],"k3":[3,4]},`
+	`"mapstruct":{"m1":{"u8":8}},`
+	`"mapptrstruct":{"m1":{"u8":8}}}`
 
+var decodedMap = map[string][]int{
+	"k1": []int{1,2,3},
+	"k2": []int{},
+	"k3": []int{3,4},
+}
 
-func _Check(t *testing.T, ok bool, name string, v interface{}) {
+var decodedMapStruct = map[string]myStruct{
+	"m1": myStruct{U8: 8},
+}
+
+var decodedMapPtrStruct = map[string]*myStruct{
+	"m1": &myStruct{U8: 8},
+}
+
+func check(t *testing.T, ok bool, name string, v interface{}) {
 	if !ok {
 		t.Errorf("%s = %v (BAD)", name, v);
 	} else {
@@ -45,36 +65,39 @@ func _Check(t *testing.T, ok bool, name string, v interface{}) {
 }
 
 func TestUnmarshal(t *testing.T) {
-	var m _MyStruct;
+	var m myStruct;
 	m.F = true;
-	ok, errtok := Unmarshal(_Encoded, &m);
+	ok, errtok := Unmarshal(encoded, &m);
 	if !ok {
 		t.Fatalf("Unmarshal failed near %s", errtok);
 	}
-	_Check(t, m.T == true, "t", m.T);
-	_Check(t, m.F == false, "f", m.F);
-	_Check(t, m.S == "abc", "s", m.S);
-	_Check(t, m.I8 == 1, "i8", m.I8);
-	_Check(t, m.I16 == 2, "i16", m.I16);
-	_Check(t, m.I32 == 3, "i32", m.I32);
-	_Check(t, m.I64 == 4, "i64", m.I64);
-	_Check(t, m.U8 == 5, "u8", m.U8);
-	_Check(t, m.U16 == 6, "u16", m.U16);
-	_Check(t, m.U32 == 7, "u32", m.U32);
-	_Check(t, m.U64 == 8, "u64", m.U64);
-	_Check(t, m.I == -9, "i", m.I);
-	_Check(t, m.U == 10, "u", m.U);
-	_Check(t, m.Fl == 11.5, "fl", m.Fl);
-	_Check(t, m.Fl32 == 12.25, "fl32", m.Fl32);
-	_Check(t, m.Fl64 == 13.75, "fl64", m.Fl64);
-	_Check(t, m.A != nil, "a", m.A);
+	check(t, m.T == true, "t", m.T);
+	check(t, m.F == false, "f", m.F);
+	check(t, m.S == "abc", "s", m.S);
+	check(t, m.I8 == 1, "i8", m.I8);
+	check(t, m.I16 == 2, "i16", m.I16);
+	check(t, m.I32 == 3, "i32", m.I32);
+	check(t, m.I64 == 4, "i64", m.I64);
+	check(t, m.U8 == 5, "u8", m.U8);
+	check(t, m.U16 == 6, "u16", m.U16);
+	check(t, m.U32 == 7, "u32", m.U32);
+	check(t, m.U64 == 8, "u64", m.U64);
+	check(t, m.I == -9, "i", m.I);
+	check(t, m.U == 10, "u", m.U);
+	check(t, m.Fl == 11.5, "fl", m.Fl);
+	check(t, m.Fl32 == 12.25, "fl32", m.Fl32);
+	check(t, m.Fl64 == 13.75, "fl64", m.Fl64);
+	check(t, m.A != nil, "a", m.A);
 	if m.A != nil {
-		_Check(t, m.A[0] == "x", "a[0]", m.A[0]);
-		_Check(t, m.A[1] == "y", "a[1]", m.A[1]);
-		_Check(t, m.A[2] == "z", "a[2]", m.A[2]);
+		check(t, m.A[0] == "x", "a[0]", m.A[0]);
+		check(t, m.A[1] == "y", "a[1]", m.A[1]);
+		check(t, m.A[2] == "z", "a[2]", m.A[2]);
 	}
-	_Check(t, m.My != nil, "my", m.My);
+	check(t, m.My != nil, "my", m.My);
 	if m.My != nil {
-		_Check(t, m.My.S == "subguy", "my.s", m.My.S);
+		check(t, m.My.S == "subguy", "my.s", m.My.S);
 	}
+	check(t, reflect.DeepEqual(m.Map, decodedMap), "map", m.Map);
+	check(t, reflect.DeepEqual(m.MapStruct, decodedMapStruct), "mapstruct", m.MapStruct);
+	check(t, reflect.DeepEqual(m.MapPtrStruct, decodedMapPtrStruct), "mapptrstruct", m.MapPtrStruct);
 }