Android 从零开始 组件 WebView

package com.vip890.guy;

import android.app.ProgressDialog;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.KeyEvent;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

/**
 * WebView
 */
public class MainActivity extends AppCompatActivity  {
    private String url = "http://2016.qq.com";
    private WebView mWebView;
    private ProgressDialog mProgressDialog;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mWebView = (WebView) findViewById(R.id.webView);
        mWebView.loadUrl(url);//加载web网页  加载本地资源文件
        mWebView.setWebViewClient(new WebViewClient(){
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
                view.loadUrl(url);
                return true;
            }
        });
        WebSettings settings = mWebView.getSettings();
        settings.setJavaScriptEnabled(true);
        settings.setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);//启用缓存
        mWebView.setWebChromeClient(new WebChromeClient(){
            @Override
            public void onProgressChanged(WebView view, int newProgress) {
                super.onProgressChanged(view, newProgress);
                if(newProgress == 100){
                    closeDialog();
                }
                else
                {
                    openDialog(newProgress);
                }
            }
        });
    }

    private void closeDialog() {
        if(mProgressDialog != null && mProgressDialog.isShowing()){
            mProgressDialog.dismiss();
            mProgressDialog = null;
        }
    }

    private void openDialog(int newProgress) {
        if(mProgressDialog == null){
            mProgressDialog=new ProgressDialog(MainActivity.this);
            mProgressDialog.setTitle("正在加载");
            mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            mProgressDialog.setProgress(newProgress);
            mProgressDialog.show();
        }else{
            mProgressDialog.setProgress(newProgress);
        }
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode == KeyEvent.KEYCODE_BACK){
            if(mWebView.canGoBack()){
                mWebView.goBack();
                return true;
            }
            else
            {
                System.exit(0);
            }
        }
        return super.onKeyDown(keyCode, event);
    }
}

//权限声明
<uses-permission android:name="android.permission.INTERNET"/>