Android SpeechRecognizer始终返回null

Question

package com.example.voicerec;

import java.util.ArrayList;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.speech.RecognitionListener;
import android.speech.RecognizerIntent;
import android.speech.SpeechRecognizer;
import android.util.Log;

public class MainActivity extends Activity {
    private SpeechRecognizer mSpeechRecognizer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mSpeechRecognizer = SpeechRecognizer.createSpeechRecognizer(this);
        mSpeechRecognizer.setRecognitionListener(new listener());
        startrecording();
    }

    public void startrecording() {
        Intent intent = new Intent(RecognizerIntent.ACTION_RECOGNIZE_SPEECH);
        intent.putExtra(RecognizerIntent.EXTRA_LANGUAGE_MODEL,
                RecognizerIntent.LANGUAGE_MODEL_FREE_FORM);
        intent.putExtra(RecognizerIntent.EXTRA_CALLING_PACKAGE,
                getPackageName());
        intent.putExtra(RecognizerIntent.EXTRA_MAX_RESULTS, 5);
        mSpeechRecognizer.startListening(intent);
        Log.i("log", "startgooglerec Started");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mSpeechRecognizer != null) {
            mSpeechRecognizer.destroy();
        }
    }

    class listener implements RecognitionListener {

        @Override
        public void onReadyForSpeech(Bundle params) {
        }

        @Override
        public void onBeginningOfSpeech() {
        }

        @Override
        public void onRmsChanged(float rmsdB) {
        }

        @Override
        public void onBufferReceived(byte[] buffer) {
        }

        @Override
        public void onEndOfSpeech() {
        }

        @Override
        public void onError(int error) {
        }

        @Override
        public void onResults(Bundle results) {

            ArrayList<String> matches = results
                    .getStringArrayList(SpeechRecognizer.RESULTS_RECOGNITION);

            System.out.println("results " + results);
            System.out.println("matches " + matches);
        }

        @Override
        public void onPartialResults(Bundle partialResults) {

        }

        @Override
        public void onEvent(int eventType, Bundle params) {

        }

    }
}

日志：

03-25 09:53:51.489: I/System.out(12149): results Bundle[{results_recognition=null, confidence_scores=null}]

SpeechRecognizer总是在onresult中返回null，即使我说话也只返回null，我也不会停止听。我能听到开始声音但不能停止声音。我的代码有什么问题，提前谢谢。