osmdroid layout inflater exception

Im trying to replace google maps with osmdroid in my application so amongst other small code changes I changed the main.xml I've replaced:

<com.google.android.maps.MapView
            android:id="@+id/mapmain"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:clickable="true"
            android:apiKey="0Me4thQTjI4jtNE1Wv51g4KNSNiojinUXncwJ6g"
            android:layout_below="@+id/filterbar"
            />

with:

<org.osmdroid.views.MapView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/mapmain"
        ></org.osmdroid.views.MapView>

now I get a runtime exception:

07-27 12:50:29.264: ERROR/AndroidRuntime(488): FATAL EXCEPTION: main
        java.lang.ExceptionInInitializerError
        at java.lang.Class.getDeclaredConstructors(Native Method)
        at java.lang.Class.getConstructor(Class.java:477)
        at android.view.开发者_如何学编程LayoutInflater.createView(LayoutInflater.java:475)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:565)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:618)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:407)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:320)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:276)
        at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:198)
        at android.app.Activity.setContentView(Activity.java:1647)
        at com.rushme.RMMapActivity.onCreate(RMMapActivity.java:60)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.access$2300(ActivityThread.java:125)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:123)
        at android.app.ActivityThread.main(ActivityThread.java:4627)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:521)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
        at dalvik.system.NativeStart.main(Native Method)
        Caused by: java.lang.NoClassDefFoundError: org.slf4j.LoggerFactory
        at org.osmdroid.views.MapView.<clinit>(MapView.java:64)
        ... 24 more

Can anybody tell me what I'm doing wrong ?