在javax.imageio.ImageIO
中,有一种方法#write(RenderedImage im, String formatName, OutputStream output)
接受开发人员希望将图像写入的“字符串型”格式。
在该文件的内部有一个称为CanEncodeImageAndFormatFilter
的内部类,它检查是否有支持上述格式的SPI(无论它是什么)。例如,BMPImageWriterSpi
声明
private static String[] formatNames = {"bmp", "BMP"};
但是在所有这些SPI类中,这些格式名称都声明为private或package-default,因此我无法在应用程序中访问它们。但是我想在我现在正在使用的API中公开该format参数,但是我不想重新发明轮子。
➥是否有枚举或可安全用作#write
方法的格式名称参数的东西?
是的,这更像是带有coding-style标签的基于意见的问题。 :D
ImageIO.write(image, "jpg", outStream);
我想要的是:
ImageIO.write(image, ImageTypes.JPEG, outStream);
默认Java API中没有您要查找的Enum。
原因是Java Image I / O API使用Service provider interface (SPI)。这允许创建可扩展的应用程序。在您的问题范围内,可以将其他图像读取器/写入器类(实现相应的接口)添加到类路径,并在运行时自动发现它们。应用程序可以使用它们,甚至在编译时都不知道它们的存在。因此,这种枚举不存在。
您可以通过以下代码段找到所有当前已知的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())
);
}
}
}
Java 13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。
假定当前目录中的以下文件
Main.class - the one from above source
ExampleImageWriterSpi.java - an example implementation of the ImageWriterSpi
ExampleImageWriterSpi.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
创建提供者配置文件(文件名指定实现的接口,该行指定实现的类)
mkdir -p META-INF/services/
echo sub.optimal.ExampleImageWriterSpi > META-INF/services/javax.imageio.spi.ImageWriterSpi
为服务提供商创建档案文件
jar cf example-image-writer.jar META-INF/ sub/
运行普通示例代码
java Main
输出(属于Java运行时的所有服务提供者)
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。但是您应该在运行时检查是否有针对此图像类型的提供程序。例如Java 9之前的Java运行时中没有TIFF。