1. 首先，需要在AndroidManifest.xml文件中为应用程序添加以下权限：

2. 然后，在Java代码中创建一个类继承自WebViewClient，重写其中的onShowCustomView和onHideCustomView方法：

public class MyCustomWebViewClient extends WebViewClient { private View mCustomView; private WebChromeClient.CustomViewCallback mCustomViewCallback; private int mOriginalSystemUiVisibility;

@Override
public void onShowCustomView(View view, WebChromeClient.CustomViewCallback callback) {
    if (mCustomView != null) {
        callback.onCustomViewHidden();
        return;
    }
    mCustomView = view;
    mOriginalSystemUiVisibility = mActivity.getWindow().getDecorView().getSystemUiVisibility();
    mCustomViewCallback = callback;
    mActivity.setContentView(view);
    mActivity.getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
    mActivity.getWindow().getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_FULLSCREEN);
}

@Override
public void onHideCustomView() {
    mActivity.getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
    mActivity.setContentView(mContentView);
    mActivity.getWindow().getDecorView().setSystemUiVisibility(mOriginalSystemUiVisibility);
    mCustomViewCallback.onCustomViewHidden();
    mCustomView = null;
}

}

3. 最后，在创建WebView对象时，将其设置为使用我们自定义的MyCustomWebViewClient：

WebView webView = new WebView(mContext); webView.setWebViewClient(new MyCustomWebViewClient());

这样就可以在Android WebView中全屏播放HTML5视频，并且横屏显示。