javajavax。图像。ImageIO文件格式常量

Question

在javax.imageio.ImageIO中，有一个方法#write(RenderedImage im, String formatName, OutputStream output)接受开发人员希望将图像写入的“字符串类型”格式

在该文件的深处有一个名为CanEncodeImageAndFormatFilter的内部类，用于检查是否存在支持上述格式的SPI（无论它是什么）。例如，BMPImageWriterSpi声明

    private static String[] formatNames = {"bmp", "BMP"};

但在所有这些SPI类中，这些格式名要么声明为private，要么声明为package default，因此我无法在应用程序中访问它们。但我想在我现在正在开发的API中公开这个格式参数，但我不想重新发明轮子

➥ 是否有一个枚举或其他可以安全地用作#write方法的格式名参数

_{是的，这更像是一个带有coding-style标记的基于意见的问题：D}

---

UPD: 我所拥有的：

ImageIO.write(image, "jpg", outStream);

我想要的是：

ImageIO.write(image, ImageTypes.JPEG, outStream);

Answer 1

在默认的Java API中没有这样的枚举

原因是Java映像I/O API使用Service provider interface (SPI)。它允许创建可扩展的应用程序。在问题的范围内，可以将其他图像读取器/写入器类（实现相应接口）添加到类路径中，并在运行时自动发现。应用程序甚至可以在编译时不知道它们的存在就使用它们。因此，这种枚举不存在

您可以通过以下代码片段发现所有当前已知的ImageWriter实现

import javax.imageio.spi.IIORegistry;
import javax.imageio.spi.ImageWriterSpi;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        IIORegistry registry = IIORegistry.getDefaultInstance();
        Iterator<ImageWriterSpi> serviceProviders = registry.getServiceProviders(ImageWriterSpi.class, false);
        while(serviceProviders.hasNext()) {
            ImageWriterSpi next = serviceProviders.next();
            System.out.printf("description: %-27s   format names: %s%n",
                    next.getDescription(Locale.ENGLISH),
                    Arrays.toString(next.getFormatNames())
            );
        }
    }
}

Java13的输出

description: Standard BMP Image Writer     format names: [bmp, BMP]
description: Standard JPEG Image Writer    format names: [JPEG, jpeg, JPG, jpg]
description: Standard WBMP Image Writer    format names: [wbmp, WBMP]
description: Standard PNG image writer     format names: [png, PNG]
description: Standard GIF image writer     format names: [gif, GIF]
description: Standard TIFF image writer    format names: [tif, TIF, tiff, TIFF]

下面是一个如何使用SPI的简化示例

假设当前目录中有以下文件

Main.class                  - the one from above source
ExampleImageWriterSpi.java  - an example implementation of the ImageWriterSpi

例如ImageWriterSpi。java

package sub.optimal;

import javax.imageio.ImageTypeSpecifier;
import javax.imageio.ImageWriter;
import javax.imageio.spi.ImageWriterSpi;
import java.io.IOException;
import java.util.Locale;

public class ExampleImageWriterSpi extends ImageWriterSpi {

    private static final String[] formatNames = { "xyz", "XYZ"};

    public String getDescription(Locale locale) {
        return "Example XYZ image writer";
    }

    @Override
    public String[] getFormatNames() {
        return formatNames;
    }

    // following is without implementation in this example
    public boolean canEncodeImage(ImageTypeSpecifier type) {
        return false;
    }

    public ImageWriter createWriterInstance(Object extension) throws IOException {
        return null;
    }
}

编译类

javac ExampleImageWriterSpi.java

创建提供商配置文件
（文件名指定实现的接口，行指定实现的类）

mkdir -p META-INF/services/
echo sub.optimal.ExampleImageWriterSpi > META-INF/services/javax.imageio.spi.ImageWriterSpi

为服务提供商创建存档

jar cf example-image-writer.jar META-INF/ sub/

运行简单的示例代码

java Main

输出（属于Java运行时的所有服务提供商）

description: Standard BMP Image Writer     format names: [bmp, BMP]
description: Standard JPEG Image Writer    format names: [JPEG, jpeg, JPG, jpg]
description: Standard WBMP Image Writer    format names: [wbmp, WBMP]
description: Standard PNG image writer     format names: [png, PNG]
description: Standard GIF image writer     format names: [gif, GIF]
description: Standard TIFF image writer    format names: [tif, TIF, tiff, TIFF]

与其他服务提供商一起运行示例

java -cp example-image-writer.jar:. Main

输出

description: Example XYZ image writer      format names: [xyz, XYZ]
description: Standard JPEG Image Writer    format names: [JPEG, jpeg, JPG, jpg]
description: Standard WBMP Image Writer    format names: [wbmp, WBMP]
description: Standard PNG image writer     format names: [png, PNG]
description: Standard GIF image writer     format names: [gif, GIF]
description: Standard BMP Image Writer     format names: [bmp, BMP]
description: Standard TIFF image writer    format names: [tif, TIF, tiff, TIFF]

您可以自己创建枚举。但您应该在运行时检查是否有可用于此图像类型的提供程序。e、 g.在Java 9之前，TIFF在Java运行时中不可用