来自资源文件夹的JavaFX图像 [英] JavaFX Image from resources folder

问题描述

由于某种原因，我一直在gradle javafx项目中获得NPE.

我的文件夹结构非常基本.我在main/java文件夹中有一个带有Java文件的程序包.我的资源也位于main/resources文件夹中.当我尝试加载image.png时，它会给我NPE.

public static Image createImage(Object context, String url) {
    Image m = null;
    InputStream str = null;
    URL _url = context.getClass().getResource(url);

    try {
        m = new Image(_url.getPath());
    } catch (NullPointerException e) {
        e.printStackTrace();
    }

    return m;
}

这是一个帮助程序类.

在Scene中，我称:Image image = Helper.createImage(this, "image.png"); 图像的绝对路径为main/resources/images/image.png.

我检查了互联网上的每个教程，但找不到任何解决方案.我还尝试使用图像的路径作为参数，并使用InputStream进行了尝试，但从未成功.

解决方案

资源

<project-dir> |--src/ |--main/ |--java/ |--resources/ |--images/ |--image.png

并且您正在使用Object和资源名称image.png调用您的方法.这里的问题是，由于要传递相对名称，因此资源相对于传递的Object的Class(即context)位于.我怀疑您的类是否位于名为images的程序包中，这意味着将找不到相对于所述类的资源.您需要传递一个绝对名称:/images/image.png.

另一个问题是您对URL#getPath()的使用.如果要找到资源，则从Class#getResource(String)获得的URL将看起来像这样:

 file:/path/to/gradle/project/build/resources/main/images/image.png
 

但是URL#getPath()的结果将给您:

 /path/to/gradle/project/build/resources/main/images/image.png
 

由于Image的工作方式，这会导致问题.来自文档:

public static Image createImage(Object context, String resourceName) { URL _url = context.getClass().getResource(resourceName); return new Image(_url.toExternalForm()); }

那么您至少有两个选择:

1. 将绝对资源名称(即"/images/image.png")传递给您的#createImage(Object,String)方法，因为image.png资源与所传递的Object(即context)不在同一个包中.

2. 将资源移到与Object中传递的类(即context)相同的包中.例如，如果上下文对象的类为com.foo.MyObject，则将资源放在src/main/resources/com/foo下，它将与MyObject放在同一包中.这将允许您继续传递相对资源名称.

当然，如Image的文档所述，您可以传递无方案的URL，并将其解释为资源名称.换句话说，您可以执行以下操作:

 Image image = new Image("images/image.png");
 

那应该可行.但是，请注意:使用模块时，只有在包含资源的软件包为opens 无条件或模块本身为open的情况下，以上方法才有效.

For some reason I keep getting an NPE in a gradle javafx project.

My folder structure is very basic. I have a package with my java files in the main/java folder. I also have my resources in the main/resources folder. When I try to load image.png it gives me an NPE.

public static Image createImage(Object context, String url) {
    Image m = null;
    InputStream str = null;
    URL _url = context.getClass().getResource(url);

    try {
        m = new Image(_url.getPath());
    } catch (NullPointerException e) {
        e.printStackTrace();
    }

    return m;
}

This is a helper class.

From the Scene I call: Image image = Helper.createImage(this, "image.png"); The absolute path to the image would be main/resources/images/image.png.

I checked every tutorial on the internet but I couldn't find any solution for this. I also tried it with the path to the image as parameter and also with an InputStream but it never worked.

解决方案

Resources

The Class#getResource(String) and related API are used for locating resources relative to the class path and/or module path. When using Class to get a resource you can pass an absolute name or a relative name. An absolute name will locate the resource relative to the root of the class path/module path; an absolute name starts with a /. A relative name will locate the resource relative to the location of the Class; a relative name does not start with a leading /.

In a typical Maven/Gradle project structure, the src/main/java and src/main/resources are roots of the class path/module path. This means all resource names are relative to those directories. It's slightly more complicated than that because the files under those directories are moved to the target/build directory and it's that location that's put on the class path/module path, but for all intents and purposes consider the source directories as the root. There's a reason a get-resource API exists in the first place, to provide an application-location-independent way of obtaining resources.

---

Issues in Your Code

From your question I gather your project structure looks something like:

<project-dir>
|--src/
   |--main/
      |--java/
      |--resources/
         |--images/
            |--image.png

And you're calling your method with an Object and a resource name of image.png. The problem here is that, since you're passing a relative name, the resource is located relative to the Class of the passed Object (i.e. context). I doubt your class is located in a package named images which means the resource will not be found relative to said class. You need to pass an absolute name: /images/image.png.

The other problem is your use of URL#getPath(). The URL you obtain from Class#getResource(String) will, if the resource were to be found, look something like this:

file:/path/to/gradle/project/build/resources/main/images/image.png

But the result of URL#getPath() will give you:

/path/to/gradle/project/build/resources/main/images/image.png

This causes a problem due to the way Image works. From the documentation:

All URLs supported by URL can be passed to the constructor. If the passed string is not a valid URL, but a path instead, the Image is searched on the classpath in that case.

Notice the second sentence. If the passed URL does not have a scheme then it's interpreted as a resource name and the Image will locate the image file relative to the classpath. In other words, since you're passing the value of URL#getPath() to the Image constructor it searches for the resource image.png in the package path.to.gradle.project.build.resources.main.images. That package does not exist. You should be passing the URL as-is to the Image constructor via URL#toString() or URL#toExternalForm().

---

Solution

If you're going to use the URL returned by Class#getResource(String) to load the Image then no matter what you need to use URL#toString() or URL#toExternalForm() instead of URL#getPath().

public static Image createImage(Object context, String resourceName) {
  URL _url = context.getClass().getResource(resourceName);
  return new Image(_url.toExternalForm());
}

Then you have at least two options:

1. Pass the absolute resource name (i.e. "/images/image.png") to your #createImage(Object,String) method since the image.png resource is not in the same package as the passed Object (i.e. context).

2. Move the resource to the same package as the class of the passed in Object (i.e. context). For instance, if the context object's class is com.foo.MyObject then place the resource under src/main/resources/com/foo and it will be in the same package as MyObject. This will allow you to continue passing the relative resource name.

Of course, as noted by the documentation of Image you can pass a scheme-less URL and it's interpreted as a resource name. In other words, you could do:

Image image = new Image("images/image.png");

And that should work. A note of caution, however: When using modules the above will only work if the resource-containing package is opens unconditionally or if the module itself is open.

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