Android Studio中使用webview显示本地html显示一片空白

Question

代码如下：

manifest:
<?xml version="1.0" encoding="utf-8"?>
package="com.example.captain991.report" >
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
android:name=".MainActivity"
android:label="@string/app_name" >

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>
<uses-permission android:name="android.permission.INTERNET"></uses-permission>

XML:
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
android:id="@+id/web_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content">

JAVA:
package com.example.captain991.report;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.Window;
import android.webkit.WebBackForwardList;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class MainActivity extends Activity {
private WebView webview;
@SuppressLint("SetJavaScriptEnabled")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
webview = (WebView) findViewById(R.id.web_view);
WebSettings webSettings = webview.getSettings();
//设置WebView属性，能够执行Javascript脚本
webSettings.setJavaScriptEnabled(true);
//设置可以访问文件
webSettings.setAllowFileAccess(true);
//设置支持缩放
webSettings.setBuiltInZoomControls(true);
//加载需要显示的网页
webSettings.setBlockNetworkImage(false);
webSettings.setBlockNetworkLoads(false);
webSettings.setDomStorageEnabled(true);
webview.loadUrl("file：///android_asset/index.htm");
//file：///android_asset/index.html
//http://www.baidu.com
//设置Web视图
webview.setWebViewClient(new webViewClient ());
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
@Override
//设置回退
//覆盖Activity类的onKeyDown(int keyCoder,KeyEvent event)方法
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK) && webview.canGoBack()) {
        webview.goBack(); //goBack()表示返回WebView的上一页面
        return true;
    }
    finish();//结束退出程序
    return false;
}

//Web视图
private class webViewClient extends WebViewClient {
    public boolean shouldOverrideUrlLoading(WebView view, String url) {
        view.loadUrl(url);
        return true;
    }
}

}