// 按返回键的时候返回上一页面不退出,goBack()表示返回webView的上一页面
    public boolean onKeyDown(int keyCoder, KeyEvent event) {
        if(!webView1.canGoBack() ){
               exit();  
                return false;
        }
        if ( keyCoder == KeyEvent.KEYCODE_BACK) {
            webView1.goBack();
               exit();  
            return false;
        }
        else{
             return super.onKeyDown(keyCoder, event);  
        }
    }
    public void exit(){  
        if (!isExit) {  
            isExit = true;  
            Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();  
            mesHandler.sendEmptyMessageDelayed(0, 2000);  
        } else {  
            Intent intent = new Intent(Intent.ACTION_MAIN);  
            intent.addCategory(Intent.CATEGORY_HOME);  
            startActivity(intent);  
            System.exit(0);  
        }  
    }