android webview点击返回键返回上一个html

@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub
		if(keyCode == KeyEvent.KEYCODE_BACK && webView.canGoBack()){
			webView.goBack();
			return true;
		}else{
			if((System.currentTimeMillis()-exitTime) > 2000){    
	            Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();                                  
	            exitTime = System.currentTimeMillis(); 
	        } else {  
	            finish();  
	        }  
		}
		return true;
	}