Always skip unexported fields when encoding

Skip creating encoders for unexported fields. They are not participating
in JSON marshaling anyway. This allows using unexported fields of
non-marshalable types in structs.

As a side-effect of this change it's no longer possible to marshal
unexported JSON fields by adding a custom type extenstion. It seems this
is desired behavior since it matches standard library and jsoniter
already disallows `json:"-"` fields from participating in custom
extensions.

Fixes #174.

feature_reflect_extension.go

@@ -227,6 +227,9 @@ func describeStruct(cfg *frozenConfig, typ reflect.Type) (*StructDescriptor, err
 	bindings := []*Binding{}
 	for i := 0; i < typ.NumField(); i++ {
 		field := typ.Field(i)
+		if unicode.IsLower([]rune(field.Name)[0]) {
+			continue
+		}
 		tag := field.Tag.Get(cfg.getTagKey())
 		tagParts := strings.Split(tag, ",")
 		if tag == "-" {

jsoniter_customize_test.go

@@ -2,11 +2,12 @@ package jsoniter
 
 import (
 	"encoding/json"
-	"github.com/stretchr/testify/require"
 	"strconv"
 	"testing"
 	"time"
 	"unsafe"
+
+	"github.com/stretchr/testify/require"
 )
 
 func Test_customize_type_decoder(t *testing.T) {
@@ -82,7 +83,7 @@ func Test_customize_field_decoder(t *testing.T) {
 }
 
 type TestObject1 struct {
-	field1 string
+	Field1 string
 }
 
 type testExtension struct {
@@ -93,7 +94,7 @@ func (extension *testExtension) UpdateStructDescriptor(structDescriptor *StructD
 	if structDescriptor.Type.String() != "jsoniter.TestObject1" {
 		return
 	}
-	binding := structDescriptor.GetField("field1")
+	binding := structDescriptor.GetField("Field1")
 	binding.Encoder = &funcEncoder{fun: func(ptr unsafe.Pointer, stream *Stream) {
 		str := *((*string)(ptr))
 		val, _ := strconv.Atoi(str)
@@ -112,7 +113,7 @@ func Test_customize_field_by_extension(t *testing.T) {
 	obj := TestObject1{}
 	err := UnmarshalFromString(`{"field-1": 100}`, &obj)
 	should.Nil(err)
-	should.Equal("100", obj.field1)
+	should.Equal("100", obj.Field1)
 	str, err := MarshalToString(obj)
 	should.Nil(err)
 	should.Equal(`{"field-1":100}`, str)

jsoniter_struct_encoder_test.go

@@ -0,0 +1,52 @@
+package jsoniter
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_encode_unexported_field(t *testing.T) {
+	type TestData struct {
+		a int
+		b <-chan int
+		C int
+		d *time.Timer
+	}
+
+	should := require.New(t)
+
+	testChan := make(<-chan int, 10)
+	testTimer := time.NewTimer(10 * time.Second)
+
+	obj := &TestData{
+		a: 42,
+		b: testChan,
+		C: 21,
+		d: testTimer,
+	}
+
+	jb, err := json.Marshal(obj)
+	should.NoError(err)
+	should.Equal([]byte(`{"C":21}`), jb)
+
+	err = json.Unmarshal([]byte(`{"a": 444, "b":"bad", "C":55, "d":{"not": "a timer"}}`), obj)
+	should.NoError(err)
+	should.Equal(42, obj.a)
+	should.Equal(testChan, obj.b)
+	should.Equal(55, obj.C)
+	should.Equal(testTimer, obj.d)
+
+	jb, err = Marshal(obj)
+	should.NoError(err)
+	should.Equal(jb, []byte(`{"C":55}`))
+
+	err = Unmarshal([]byte(`{"a": 444, "b":"bad", "C":256, "d":{"not":"a timer"}}`), obj)
+	should.NoError(err)
+	should.Equal(42, obj.a)
+	should.Equal(testChan, obj.b)
+	should.Equal(256, obj.C)
+	should.Equal(testTimer, obj.d)
+}