diff --git a/container.go b/container.go
new file mode 100644
index 0000000..6a6d2f6
--- /dev/null
+++ b/container.go
@@ -0,0 +1,91 @@
+package di
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"sync"
+)
+
+type container struct {
+	mu        sync.RWMutex
+	creators  map[string]func() any
+	instances map[string]any
+}
+
+var globalContainer *container
+var once sync.Once
+
+func getContainer() *container {
+	once.Do(func() {
+		globalContainer = &container{
+			mu:        sync.RWMutex{},
+			creators:  make(map[string]func() any),
+			instances: make(map[string]any),
+		}
+	})
+	return globalContainer
+}
+
+func (c *container) injectable(typ reflect.Type, creator func() any) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	selector := c.getSelector(typ)
+	c.creators[selector] = creator
+}
+
+func (c *container) replace(typ reflect.Type, creator func() any, identifier ...string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	selector := c.getSelector(typ)
+	instanceSelector := c.getSelector(typ, identifier...)
+	c.creators[selector] = creator
+	createdInstance := creator()
+	c.instances[instanceSelector] = createdInstance
+}
+
+func (c *container) inject(typ reflect.Type, identifier ...string) (any, bool) {
+	instanceSelector := c.getSelector(typ, identifier...)
+
+	c.mu.RLock()
+	if instance, ok := c.instances[instanceSelector]; ok {
+		c.mu.RUnlock()
+		return instance, true
+	}
+	c.mu.RUnlock()
+
+	c.mu.RLock()
+	if instance, ok := c.instances[instanceSelector]; ok {
+		c.mu.RUnlock()
+		return instance, true
+	}
+	c.mu.RUnlock()
+
+	selector := c.getSelector(typ)
+	creator, creatorExists := c.creators[selector]
+	if !creatorExists {
+		return nil, false
+	}
+
+	createdInstance := creator()
+	c.mu.Lock()
+	c.instances[instanceSelector] = createdInstance
+	c.mu.Unlock()
+	return createdInstance, true
+}
+
+func (c *container) destroy(typ reflect.Type, identifier ...string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	instanceSelector := c.getSelector(typ, identifier...)
+	delete(c.instances, instanceSelector)
+}
+
+func (c *container) getSelector(typ reflect.Type, identifier ...string) string {
+	typeName := typ.String()
+	additionalKey := strings.Join(identifier, "_")
+	return fmt.Sprintf("%s_%s", typeName, additionalKey)
+}
diff --git a/injector.go b/injector.go
index feaaabb..2bd8fe5 100644
--- a/injector.go
+++ b/injector.go
@@ -1,83 +1,37 @@
 package di
 
 import (
-	"fmt"
 	"reflect"
-	"strings"
-	"sync"
 )
 
-var creatorMutex = sync.Mutex{}
-var instanceMutex = sync.Mutex{}
-var creators = make(map[string]func() any)
-var instances = make(map[string]any)
-
 // Injectable marks a constructor Function of a Struct for DI
 func Injectable[T any](creator func() T) {
-	creatorMutex.Lock()
-	defer creatorMutex.Unlock()
-	creators[getSelector[T]()] = func() any {
-		return creator()
-	}
+	typ := reflect.TypeOf((*T)(nil)).Elem()
+	getContainer().injectable(typ, func() any { return creator() })
 }
 
 func Replace[T any](creator func() T, identifier ...string) {
-	Injectable(creator)
-	selector := getSelector[T]()
-	instanceSelector := getSelector[T](identifier...)
-	cre, creatorExists := creators[selector]
-	if !creatorExists {
-		return
-	}
-	createdInstance, instanceCreated := cre().(T)
-	if instanceCreated {
-		instanceMutex.Lock()
-		defer instanceMutex.Unlock()
-		instances[instanceSelector] = createdInstance
-	}
+	typ := reflect.TypeOf((*T)(nil)).Elem()
+	getContainer().replace(typ, func() any { return creator() })
+}
+
+func ReplaceInstance[T any](instance T, identifier ...string) {
+	Replace(func() T { return instance }, identifier...)
 }
 
 // Inject gets or create a Instance of the Struct used the Injectable constructor Function
 func Inject[T any](identifier ...string) T {
-	var nilResult T
-	selector := getSelector[T]()
-	instanceSelector := getSelector[T](identifier...)
-	_, instanceExists := instances[instanceSelector].(T)
-	if !instanceExists {
-		creator, creatorExists := creators[selector]
-		if !creatorExists {
-			return nilResult
-		}
-		createdInstance, instanceCreated := creator().(T)
-		if instanceCreated {
-			instanceMutex.Lock()
-			defer instanceMutex.Unlock()
-			instance, instanceExists := instances[instanceSelector].(T)
-			if instanceExists {
-				return instance
-			}
-			instances[instanceSelector] = createdInstance
+	var result T
+	typ := reflect.TypeOf((*T)(nil)).Elem()
+	if instance, ok := getContainer().inject(typ, identifier...); ok {
+		if result, ok = instance.(T); ok {
+			return result
 		}
 	}
-	return instances[instanceSelector].(T)
+	return result
 }
 
 func Destroy[T any](identifier ...string) {
-	instanceMutex.Lock()
-	defer instanceMutex.Unlock()
-	instanceSelector := getSelector[T](identifier...)
-	delete(instances, instanceSelector)
-}
-
-func getSelector[T any](identifier ...string) string {
-	var def T
-	typeName := ""
-	typeOf := reflect.TypeOf(def)
-	if typeOf != nil {
-		typeName = typeOf.String()
-	} else {
-		typeName = reflect.TypeOf((*T)(nil)).Elem().String()
-	}
-	additionalKey := strings.Join(identifier, "_")
-	return fmt.Sprintf("%s_%s", typeName, additionalKey)
+	typ := reflect.TypeOf((*T)(nil)).Elem()
+	getContainer().destroy(typ, identifier...)
 }
diff --git a/injector_test.go b/injector_test.go
index dc8710c..40f4959 100644
--- a/injector_test.go
+++ b/injector_test.go
@@ -116,6 +116,9 @@ func (ctx *messageService) GetTextServiceID() string {
 }
 
 func TestInject(t *testing.T) {
+	// testMutex.Lock()
+	// defer testMutex.Unlock()
+
 	msg := newMessageService()
 	println(msg.texts.Greeting())
 	if msg.texts.Greeting() != "Hello Markus" {
@@ -124,6 +127,9 @@ func TestInject(t *testing.T) {
 }
 
 func TestInject_Duplicate(t *testing.T) {
+	// testMutex.Lock()
+	// defer testMutex.Unlock()
+
 	msg1 := newMessageService()
 	msg2 := newMessageService()
 	println(msg1.texts.Greeting())
@@ -192,6 +198,19 @@ func TestOverwriteInjectable(t *testing.T) {
 	}
 }
 
+func TestOverwriteInjectableInstance(t *testing.T) {
+	basic := di.Inject[overridableService]()
+	basicID := basic.GetInstanceID()
+	di.ReplaceInstance(newBasicOverridableServiceMock())
+	basic = di.Inject[overridableService]()
+	if basic.GetInstanceID() == basicID {
+		t.Errorf("basic and newOne are the same instance")
+	}
+	if basic.GetValue() != "i am mock" {
+		t.Errorf("service not overwritten")
+	}
+}
+
 func TestDestroy(t *testing.T) {
 	_ = di.Inject[textService]("a")
 	di.Destroy[textService]("a")
diff --git a/makefile b/makefile
new file mode 100644
index 0000000..135ef0a
--- /dev/null
+++ b/makefile
@@ -0,0 +1,2 @@
+test:
+	- go test ./...
\ No newline at end of file