java.lang.NullPointerException using static WebDriver instance

问题

I want to separate my code in to smaller functions. But had an issue as driver was not available to all functions. So i declared it as a constant (or is there a better way of doing this ?)

but in 3rd function it is failing on line :

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

Here is the console message :

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);
    }
}

回答1:

The error says it all :

Exception in thread "main" java.lang.NullPointerException

In class scrapeit you have already declared webdriver as an instance of WebDriver as static as follows :

static WebDriver driver;

But then you are again initializing another driver as a new instance of WebDriver as follows :

WebDriver driver = new ChromeDriver();

Hence you see java.lang.NullPointerException

Solution

A quick solution will instead of creating another instance of the WebDriver you need to use the static instance of WebDriver. So you need to remove WebDriver from WebDriver driver = new ChromeDriver(); as follows :

driver = new ChromeDriver();

来源：https://stackoverflow.com/questions/49150234/java-lang-nullpointerexception-using-static-webdriver-instance