diff --git a/quartz/job.go b/quartz/job.go
index 2ec2dcc..d6cce0e 100644
--- a/quartz/job.go
+++ b/quartz/job.go
@@ -119,11 +119,11 @@ func (sh *ShellJob) Execute(ctx context.Context) {
 	} else {
 		sh.jobStatus = OK
 	}
+	sh.Unlock()
 
 	if sh.callback != nil {
 		sh.callback(ctx, sh)
 	}
-	sh.Unlock()
 }
 
 // ExitCode returns the exit code of the ShellJob.
@@ -187,6 +187,9 @@ func NewCurlJob(request *http.Request) *CurlJob {
 
 // NewCurlJobWithOptions returns a new CurlJob configured with CurlJobOptions.
 func NewCurlJobWithOptions(request *http.Request, opts CurlJobOptions) *CurlJob {
+	if opts.HTTPClient == nil {
+		opts.HTTPClient = http.DefaultClient
+	}
 	return &CurlJob{
 		httpClient:  opts.HTTPClient,
 		request:     request,
@@ -240,8 +243,6 @@ func formatRequest(r *http.Request) string {
 // Execute is called by a Scheduler when the Trigger associated with this job fires.
 func (cu *CurlJob) Execute(ctx context.Context) {
 	cu.Lock()
-	defer cu.Unlock()
-
 	cu.request = cu.request.WithContext(ctx)
 	var err error
 	cu.response, err = cu.httpClient.Do(cu.request)
@@ -251,6 +252,7 @@ func (cu *CurlJob) Execute(ctx context.Context) {
 	} else {
 		cu.jobStatus = FAILURE
 	}
+	cu.Unlock()
 
 	if cu.callback != nil {
 		cu.callback(ctx, cu)
diff --git a/quartz/job_test.go b/quartz/job_test.go
index 68a0c70..b4efd4b 100644
--- a/quartz/job_test.go
+++ b/quartz/job_test.go
@@ -254,3 +254,37 @@ func TestShellJob_Execute(t *testing.T) {
 	assertEqual(t, 127, sh.ExitCode())
 	// the return value is different under different platforms.
 }
+
+func TestShellJob_WithCallback(t *testing.T) {
+	stdoutShell := "echo -n ok"
+	resultChan := make(chan string, 1)
+	shJob := quartz.NewShellJobWithCallback(
+		stdoutShell,
+		func(_ context.Context, job *quartz.ShellJob) {
+			resultChan <- job.Stdout()
+		},
+	)
+	shJob.Execute(context.Background())
+
+	assertEqual(t, "", shJob.Stderr())
+	assertEqual(t, "ok", shJob.Stdout())
+	assertEqual(t, "ok", <-resultChan)
+}
+
+func TestCurlJob_WithCallback(t *testing.T) {
+	request, err := http.NewRequest(http.MethodGet, worldtimeapiURL, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resultChan := make(chan quartz.JobStatus, 1)
+	opts := quartz.CurlJobOptions{
+		Callback: func(_ context.Context, job *quartz.CurlJob) {
+			resultChan <- job.JobStatus()
+		},
+	}
+	curlJob := quartz.NewCurlJobWithOptions(request, opts)
+	curlJob.Execute(context.Background())
+
+	assertEqual(t, 466866822, curlJob.Key())
+	assertEqual(t, quartz.OK, <-resultChan)
+}
diff --git a/quartz/scheduler_test.go b/quartz/scheduler_test.go
index 86f5c51..34fd997 100644
--- a/quartz/scheduler_test.go
+++ b/quartz/scheduler_test.go
@@ -37,6 +37,7 @@ func TestScheduler(t *testing.T) {
 	jobKeys[3] = errCurlJob.Key()
 
 	sched.Start(ctx)
+	assertEqual(t, sched.IsStarted(), true)
 	sched.ScheduleJob(ctx, shellJob, quartz.NewSimpleTrigger(time.Millisecond*800))
 	sched.ScheduleJob(ctx, curlJob, quartz.NewRunOnceTrigger(time.Millisecond))
 	sched.ScheduleJob(ctx, errShellJob, quartz.NewRunOnceTrigger(time.Millisecond))
@@ -47,14 +48,17 @@ func TestScheduler(t *testing.T) {
 	assertEqual(t, scheduledJobKeys, []int{3668896347, 2787962474})
 
 	_, err = sched.GetScheduledJob(jobKeys[0])
-	if err != nil {
-		t.Fail()
-	}
+	assertEqual(t, err, nil)
 
 	err = sched.DeleteJob(shellJob.Key())
-	if err != nil {
-		t.Fail()
-	}
+	assertEqual(t, err, nil)
+
+	nonExistentJobKey := 1111
+	_, err = sched.GetScheduledJob(nonExistentJobKey)
+	assertNotEqual(t, err, nil)
+
+	err = sched.DeleteJob(nonExistentJobKey)
+	assertNotEqual(t, err, nil)
 
 	scheduledJobKeys = sched.GetJobKeys()
 	assertEqual(t, len(scheduledJobKeys), 1)
diff --git a/quartz/trigger_test.go b/quartz/trigger_test.go
index 7955c47..857ac75 100644
--- a/quartz/trigger_test.go
+++ b/quartz/trigger_test.go
@@ -35,6 +35,7 @@ func TestRunOnceTrigger(t *testing.T) {
 	assertEqual(t, err, nil)
 
 	next, err = trigger.NextFireTime(next)
+	trigger.Description()
 	assertEqual(t, next, 0)
 	assertNotEqual(t, err, nil)
 }