使用静态 WebDriver 实例的 java.lang.NullPointerException

我想将我的代码分成更小的函数。 但有一个问题，因为驱动程序并非适用于所有功能。 所以我将它声明为常量（或者有更好的方法吗？）

但在第三个函数中，它在线失败：

Select dropdown_finance_product = new Select(driver.findElement(By.xpath("//select[@id='ResultsNumber']")));

这是控制台消息：

Exception in thread "main" java.lang.NullPointerException
    at Scraping.scrapeit.fetch_urls(scrapeit.java:49)
    at Scraping.scrapeit.main(scrapeit.java:24)

Code :

package Scraping;

import java.io.IOException;
import java.util.List;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.support.ui.ExpectedConditions;
import org.openqa.selenium.support.ui.Select;
import org.openqa.selenium.support.ui.WebDriverWait;

import tools.Xls_Reader;

public class scrapeit {

    static WebDriver driver;

    public static void main(String[] args) throws IOException {

        start_browser();
        fetch_urls();
        read_excel();
    }

    public static void start_browser() {

        System.setProperty("webdriver.chrome.driver", "C:\\Chrome Driver\\chromedriver.exe");
        WebDriver driver = new ChromeDriver();

        driver.manage().deleteAllCookies();
        driver.get("http://www.example.com/search/items/new/");
        driver.manage().window().maximize();
        WebDriverWait wait = new WebDriverWait(driver, 20);
        wait.until(ExpectedConditions.titleContains("New items));
    }

    public static void fetch_urls() {

        Select dropdown_finance_product = new Select(driver.findElement(By.xpath("//select[@id='ResultsNumber']")));
        dropdown_finance_product.selectByVisibleText("100");
        System.out.println("Selected 100 Items Dropdown");

        // Open Excel and write urls back
        Xls_Reader datatable = new Xls_Reader("C:\\scrape.xlsx");

        List<WebElement> num_items = driver.findElements(
                By.xpath("//a [contains(@href,'http://www.example.com/search/items/new/latest-')] "));

        for (int i = 0; i < num_items.size(); i++) {
            System.out.println("How many items on page = : " + num_items.size() + " counter = " + i);

            String link = num_items.get(i).getAttribute("href");

            datatable.setCellData("New", "URL", i + 2, link);
            System.out.println("URL : " + link);
        }
    }

    public static void read_excel() {

        // Read in url and process...
        Xls_Reader datatable = new Xls_Reader("C:\\scrape.xlsx");
        int r = datatable.getRowCount("URL");
        int c = datatable.getColumnCount("URL");
        System.out.println("num of rows = " + r + " num of cols = " + c);
    }
}

该错误说明了一切：

Exception in thread "main" java.lang.NullPointerException

In class scrapeit你已经声明了网络驱动程序作为一个例子网络驱动程序 as static如下 ：

static WebDriver driver;

但随后你再次初始化另一个driver作为一个新实例网络驱动程序如下 ：

WebDriver driver = new ChromeDriver();

因此你看到java.lang.NullPointerException

Solution

一个快速的解决方案将不是创建另一个实例网络驱动程序你需要使用static的实例网络驱动程序。所以你需要删除网络驱动程序 from WebDriver driver = new ChromeDriver();如下 ：

driver = new ChromeDriver();