diff --git a/rpcx/internal/balancer/p2c/p2c_test.go b/rpcx/internal/balancer/p2c/p2c_test.go
index 2ca1e6c63..5b8a2d3dc 100644
--- a/rpcx/internal/balancer/p2c/p2c_test.go
+++ b/rpcx/internal/balancer/p2c/p2c_test.go
@@ -4,7 +4,9 @@ import (
 	"context"
 	"fmt"
 	"strconv"
+	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/tal-tech/go-zero/core/logx"
@@ -38,6 +40,10 @@ func TestP2cPicker_Pick(t *testing.T) {
 			name:       "single",
 			candidates: 1,
 		},
+		{
+			name:       "two",
+			candidates: 2,
+		},
 		{
 			name:       "multiple",
 			candidates: 100,
@@ -46,6 +52,7 @@ func TestP2cPicker_Pick(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
+			const total = 10000
 			builder := new(p2cPickerBuilder)
 			ready := make(map[resolver.Address]balancer.SubConn)
 			for i := 0; i < test.candidates; i++ {
@@ -55,7 +62,9 @@ func TestP2cPicker_Pick(t *testing.T) {
 			}
 
 			picker := builder.Build(ready)
-			for i := 0; i < 10000; i++ {
+			var wg sync.WaitGroup
+			wg.Add(total)
+			for i := 0; i < total; i++ {
 				_, done, err := picker.Pick(context.Background(), balancer.PickInfo{
 					FullMethodName: "/",
 					Ctx:            context.Background(),
@@ -64,11 +73,16 @@ func TestP2cPicker_Pick(t *testing.T) {
 				if i%100 == 0 {
 					err = status.Error(codes.DeadlineExceeded, "deadline")
 				}
-				done(balancer.DoneInfo{
-					Err: err,
-				})
+				go func() {
+					time.Sleep(time.Millisecond)
+					done(balancer.DoneInfo{
+						Err: err,
+					})
+					wg.Done()
+				}()
 			}
 
+			wg.Wait()
 			dist := make(map[interface{}]int)
 			conns := picker.(*p2cPicker).conns
 			for _, conn := range conns {
diff --git a/rpcx/internal/resolver/directbuilder.go b/rpcx/internal/resolver/directbuilder.go
index dd30a5e97..2db89d203 100644
--- a/rpcx/internal/resolver/directbuilder.go
+++ b/rpcx/internal/resolver/directbuilder.go
@@ -11,7 +11,9 @@ type directBuilder struct{}
 func (d *directBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (
 	resolver.Resolver, error) {
 	var addrs []resolver.Address
-	endpoints := strings.Split(target.Endpoint, EndpointSep)
+	endpoints := strings.FieldsFunc(target.Endpoint, func(r rune) bool {
+		return r == EndpointSep
+	})
 
 	for _, val := range subset(endpoints, subsetSize) {
 		addrs = append(addrs, resolver.Address{
diff --git a/rpcx/internal/resolver/directbuilder_test.go b/rpcx/internal/resolver/directbuilder_test.go
new file mode 100644
index 000000000..2aebc8e45
--- /dev/null
+++ b/rpcx/internal/resolver/directbuilder_test.go
@@ -0,0 +1,52 @@
+package resolver
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/tal-tech/go-zero/core/lang"
+	"github.com/tal-tech/go-zero/core/mathx"
+	"google.golang.org/grpc/resolver"
+)
+
+func TestDirectBuilder_Build(t *testing.T) {
+	tests := []int{
+		0,
+		1,
+		2,
+		subsetSize / 2,
+		subsetSize,
+		subsetSize * 2,
+	}
+
+	for _, test := range tests {
+		t.Run(strconv.Itoa(test), func(t *testing.T) {
+			var servers []string
+			for i := 0; i < test; i++ {
+				servers = append(servers, fmt.Sprintf("localhost:%d", i))
+			}
+			var b directBuilder
+			cc := new(mockedClientConn)
+			_, err := b.Build(resolver.Target{
+				Scheme:   DirectScheme,
+				Endpoint: strings.Join(servers, ","),
+			}, cc, resolver.BuildOptions{})
+			assert.Nil(t, err)
+			size := mathx.MinInt(test, subsetSize)
+			assert.Equal(t, size, len(cc.state.Addresses))
+			m := make(map[string]lang.PlaceholderType)
+			for _, each := range cc.state.Addresses {
+				m[each.Addr] = lang.Placeholder
+			}
+			assert.Equal(t, size, len(m))
+		})
+	}
+}
+
+func TestDirectBuilder_Scheme(t *testing.T) {
+	var b directBuilder
+	assert.Equal(t, DirectScheme, b.Scheme())
+}
diff --git a/rpcx/internal/resolver/discovbuilder.go b/rpcx/internal/resolver/discovbuilder.go
index e770a78b4..103e94195 100644
--- a/rpcx/internal/resolver/discovbuilder.go
+++ b/rpcx/internal/resolver/discovbuilder.go
@@ -11,7 +11,9 @@ type discovBuilder struct{}
 
 func (d *discovBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (
 	resolver.Resolver, error) {
-	hosts := strings.Split(target.Authority, EndpointSep)
+	hosts := strings.FieldsFunc(target.Authority, func(r rune) bool {
+		return r == EndpointSep
+	})
 	sub, err := discov.NewSubscriber(hosts, target.Endpoint)
 	if err != nil {
 		return nil, err
diff --git a/rpcx/internal/resolver/resolver.go b/rpcx/internal/resolver/resolver.go
index 635a8615a..d1622358e 100644
--- a/rpcx/internal/resolver/resolver.go
+++ b/rpcx/internal/resolver/resolver.go
@@ -5,7 +5,7 @@ import "google.golang.org/grpc/resolver"
 const (
 	DirectScheme = "direct"
 	DiscovScheme = "discov"
-	EndpointSep  = ","
+	EndpointSep  = ','
 	subsetSize   = 32
 )
 
diff --git a/rpcx/internal/resolver/resolver_test.go b/rpcx/internal/resolver/resolver_test.go
new file mode 100644
index 000000000..eac31b806
--- /dev/null
+++ b/rpcx/internal/resolver/resolver_test.go
@@ -0,0 +1,36 @@
+package resolver
+
+import (
+	"testing"
+
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+func TestNopResolver(t *testing.T) {
+	// make sure ResolveNow & Close don't panic
+	var r nopResolver
+	r.ResolveNow(resolver.ResolveNowOptions{})
+	r.Close()
+}
+
+type mockedClientConn struct {
+	state resolver.State
+}
+
+func (m *mockedClientConn) UpdateState(state resolver.State) {
+	m.state = state
+}
+
+func (m *mockedClientConn) ReportError(err error) {
+}
+
+func (m *mockedClientConn) NewAddress(addresses []resolver.Address) {
+}
+
+func (m *mockedClientConn) NewServiceConfig(serviceConfig string) {
+}
+
+func (m *mockedClientConn) ParseServiceConfig(serviceConfigJSON string) *serviceconfig.ParseResult {
+	return nil
+}
diff --git a/rpcx/internal/target.go b/rpcx/internal/target.go
index d401472c1..52c7b2a72 100644
--- a/rpcx/internal/target.go
+++ b/rpcx/internal/target.go
@@ -8,10 +8,11 @@ import (
 )
 
 func BuildDirectTarget(endpoints []string) string {
-	return fmt.Sprintf("%s:///%s", resolver.DirectScheme, strings.Join(endpoints, resolver.EndpointSep))
+	return fmt.Sprintf("%s:///%s", resolver.DirectScheme, strings.Join(
+		endpoints, fmt.Sprint(resolver.EndpointSep)))
 }
 
 func BuildDiscovTarget(endpoints []string, key string) string {
-	return fmt.Sprintf("%s://%s/%s", resolver.DiscovScheme,
-		strings.Join(endpoints, resolver.EndpointSep), key)
+	return fmt.Sprintf("%s://%s/%s", resolver.DiscovScheme, strings.Join(
+		endpoints, fmt.Sprint(resolver.EndpointSep)), key)
 }