android – 为什么onPostExecute在doInBackground在AsyncTask中完成之前执行

我试图创建一个AsyncTask来从服务器端获取一些数据.但是它在onPostExecute()的行中报告了一个Null指针异常“IoUtils.copy((response_.getEntity().getContent()),writer);”我使用一些日志来查看发生了什么,发现日志1和3被打印但log 2没有.为什么在client.executed()完成之前执行了onPostExecute？有人可以给点建议吗？

private class GetForm extends AsyncTask<String,Integer,Object> {
    private httpResponse response_;
    private Exception exception_;
    private String url_;

    public GetForm(String url) {
        super();
        url_ = url;
    }
    protected Object doInBACkground(String... params) {
        try {
            Log.i("mylog","inside doInBACkground 1.");
            httpClient client = new DefaulthttpClient();
            httpGet request = new httpGet(url_);
            response_ = client.execute(request);
                            Log.i("mylog","inside doInBACkground 2.");

        } catch (Exception E) {
            exception_ = e;
        }
        return response_;
    }
    @Override
    protected void onPostExecute(Object o) {
        // Your current UI stuff here.
        Log.i("mylog","inside onPostExecute 3.");
        StringWriter writer = new StringWriter();
        try{
            IoUtils.copy((response_.getEntity().getContent()),writer);
        ......

        }catch(IOException E) {
            e.printStackTrace();
        }
    }
}