JAVA快速精准获取图片类型及实现源码分析 博客分类： JAVA 快速精准获取图片信息类型ImageInputStreamReaderSpi
关于在Java 程序中如何根据上传的图片流信息，获取上传的图片类型、宽、高这个问题一直纠结着我，终于抽出时间对这个问题分析
在JDK 中有提供现成的API 来查询图片的类型、宽、高。
ImageInputStream imageInputStream = ImageIO.createImageInputStream(new File("D:\1.jpg"));
Iterator<ImageReader> iter = ImageIO.getImageReaders(imageInputStream);
if (null != iter && iter.hasNext()) {
ImageReader reader = iter.next();
format = reader.getFormatName(); //获得图片的类型
reader.setInput(imageInputStream, true);
reader.getWidth(0); //获得图片的宽
reader.getHeight(0); //获得图片的高
}
获取图片类型
重要源码分析
1、ImageIO.getImageReaders()方法，参数是ImageInputStream 或者其他。
/**
* Returns an <code>Iterator</code> containing all currently
* registered <code>ImageReader</code>s that claim to be able to
* decode the supplied <code>Object</code>, typically an
* <code>ImageInputStream</code>.
*
* <p> The stream position is left at its prior position upon
* exit from this method.
*
* @param input an <code>ImageInputStream</code> or other
* <code>Object</code> containing encoded image data.
*
* @return an <code>Iterator</code> containing <code>ImageReader</code>s.
*
* @exception IllegalArgumentException if <code>input</code> is
* <code>null</code>.
*
* @see javax.imageio.spi.ImageReaderSpi#canDecodeInput
*/
public static Iterator<ImageReader> getImageReaders(Object input) {
if (input == null) {
throw new IllegalArgumentException("input == null!");
}
Iterator iter;
// Ensure category is present
try {
iter = theRegistry.getServiceProviders(ImageReaderSpi.class,
new CanDecodeInputFilter(input),
true); //读取项目
} catch (IllegalArgumentException e) {
return new HashSet().iterator();
}
return new ImageReaderIterator(iter);
}
仔细观察ImageReader对象发现它是一个抽象类，在抽象类中存在一个抽象的属性ImageReaderSpi originatingProvider 。其实这个属性的实现类具有以下几种继承类：
BMPImageReaderSpi 、GIFImageReaderSpi、JPEGImageReaderSpi、PNGImageReaderSpi、WBMPImageReaderSpi ，这个抽象属性就是读取图片流信息的关键。
1.1 IIORegistry.getServiceProviders()方法用以获得ImageReader的确切实现类，参数1：抽象类的源文件，用于获取继承抽象类的集合，参数2：对继承抽象类的集合的过滤器CanDecodeInputFilter，参数3：boolean型 返回的结果是否是Key/Value返回结果。
/**
* Returns an <code>Iterator</code> containing service provider
* objects within a given category that satisfy a criterion
* imposed by the supplied <code>ServiceRegistry.Filter</code>
* object's <code>filter</code> method.
*
* <p> The <code>useOrdering</code> argument controls the
* ordering of the results using the same rules as
* <code>getServiceProviders(Class, boolean)</code>.
*
* @param category the category to be retrieved from.
* @param filter an instance of <code>ServiceRegistry.Filter</code>
* whose <code>filter</code> method will be invoked.
* @param useOrdering <code>true</code> if pairwise orderings
* should be taken account in ordering the returned objects.
*
* @return an <code>Iterator</code> containing service provider
* objects from the given category, possibly in order.
*
* @exception IllegalArgumentException if there is no category
* corresponding to <code>category</code>.
*/
public <T> Iterator<T> getServiceProviders(Class<T> category,
Filter filter,
boolean useOrdering) {
SubRegistry reg = (SubRegistry)categoryMap.get(category);
if (reg == null) {
throw new IllegalArgumentException("category unknown!");
}
Iterator iter = getServiceProviders(category, useOrdering);
return new FilterIterator(iter, filter);
}
getServiceProviders()方法用以获得ImageReaderSpi继承子类的迭代器BMPImageReaderSpi 、GIFImageReaderSpi、JPEGImageReaderSpi、PNGImageReaderSpi、WBMPImageReaderSpi的集合，这个方法是公用方法，暂未研究透彻因此不在这里丢人。
new FilterIterator(iter, filter);是调用传进来的CanDecodeInputFilter过滤器的filter方法进行过滤。代码如下：
public FilterIterator(Iterator<T> iter,
ServiceRegistry.Filter filter) {
this.iter = iter;
this.filter = filter;
advance();
}
private void advance() {
while (iter.hasNext()) {
T elt = iter.next();
if (filter.filter(elt)) {
next = elt;
return;
}
}
next = null;
}
filter.filter()方法如下所示：
static class CanDecodeInputFilter
implements ServiceRegistry.Filter {
Object input;
public CanDecodeInputFilter(Object input) {
this.input = input;
}
public boolean filter(Object elt) {
try {
ImageReaderSpi spi = (ImageReaderSpi)elt;
ImageInputStream stream = null;
if (input instanceof ImageInputStream) {
stream = (ImageInputStream)input;
}
// Perform mark/reset as a defensive measure
// even though plug-ins are supposed to take
// care of it.
boolean canDecode = false;
if (stream != null) {
stream.mark();
}
canDecode = spi.canDecodeInput(input);
if (stream != null) {
stream.reset();
}
return canDecode;
} catch (IOException e) {
return false;
}
}
}
CanDecodeInputFilter 过滤器会对ImageReaderSpi子类集合进行迭代循环并调用各自的重写canDecodeInput方法对stream进行检测是那种类型(stream是在new CanDecodeInputFilter时传输进来的ImageInputStream)。
接下来我们来看下ImageReaderSpi的子类中对canDecodeInput方法的内容
1)BMPImageReaderSpi.canDecodeInput
public boolean canDecodeInput(Object source) throws IOException {
if (!(source instanceof ImageInputStream)) {
return false;
}
ImageInputStream stream = (ImageInputStream)source;
byte[] b = new byte[2];
stream.mark();
stream.readFully(b);
stream.reset();
return (b[0] == 0x42) && (b[1] == 0x4d);
}
2)GIFImageReaderSpi.canDecodeInput
public boolean canDecodeInput(Object input) throws IOException {
if (!(input instanceof ImageInputStream)) {
return false;
}
ImageInputStream stream = (ImageInputStream)input;
byte[] b = new byte[6];
stream.mark();
stream.readFully(b);
stream.reset();
return b[0] == 'G' && b[1] == 'I' && b[2] == 'F' && b[3] == '8' &&
(b[4] == '7' || b[4] == '9') && b[5] == 'a';
}
3)JPEGImageReaderSpi.canDecodeInput
public boolean canDecodeInput(Object source) throws IOException {
if (!(source instanceof ImageInputStream)) {
return false;
}
ImageInputStream iis = (ImageInputStream) source;
iis.mark();
// If the first two bytes are a JPEG SOI marker, it's probably
// a JPEG file. If they aren't, it definitely isn't a JPEG file.
int byte1 = iis.read();
int byte2 = iis.read();
iis.reset();
if ((byte1 == 0xFF) && (byte2 == JPEG.SOI)) {
return true;
}
return false;
}
4)PNGImageReaderSpi.canDecodeInput
public boolean canDecodeInput(Object source) throws IOException {
if (!(source instanceof ImageInputStream)) {
return false;
}
ImageInputStream stream = (ImageInputStream)source;
byte[] b = new byte[3];
stream.mark();
stream.readFully(b);
stream.reset();
return ((b[0] == (byte)0) && // TypeField == 0
b[1] == 0 && // FixHeaderField == 0xxx00000; not support ext header
((b[2] & 0x8f) != 0 || (b[2] & 0x7f) != 0)); // First width byte
//XXX: b[2] & 0x8f) != 0 for the bug in Sony Ericsson encoder.
}
5)WBMPImageReaderSpi.canDecodeInput
public boolean canDecodeInput(Object source) throws IOException {
if (!(source instanceof ImageInputStream)) {
return false;
}
ImageInputStream stream = (ImageInputStream)source;
byte[] b = new byte[3];
stream.mark();
stream.readFully(b);
stream.reset();
return ((b[0] == (byte)0) && // TypeField == 0
b[1] == 0 && // FixHeaderField == 0xxx00000; not support ext header
((b[2] & 0x8f) != 0 || (b[2] & 0x7f) != 0)); // First width byte
//XXX: b[2] & 0x8f) != 0 for the bug in Sony Ericsson encoder.
}
重点说明：看到这里就应该明白了，相同图片类型的图片流内容中的固定多少位到多少位将会是相同的，具体图盘类型的公式可以如上所示。
1.2 new ImageReaderIterator(iter)方法，只是将解析出来的ImageReaderSpi 赋值到ImageReader对象的 originatingProvider 属性中。
2、ImageReader.getFormatName(); 方法用以读出图片的类型，
public String getFormatName() throws IOException {
return originatingProvider.getFormatNames()[0];
}
上面的1如果看懂了这里就非常好理解了，只是调用第一步中获得ImageReaderSpi 子类中getFormatNames方法默认取第一个值。
说到这里我们就可以自己总结出一个公用方法来根据ImageInputStream来获取图片的类型信息(当然方法作用不大，不推荐使用，只是用于学习)。
返回的就是图片的类型信息
private static String checkImageType(ImageInputStream stream){
String fileType="jpg";
byte[] b;
try {
b = new byte[8];
stream.mark();
stream.readFully(b);
stream.reset();
int byte1 = stream.read();
int byte2 = stream.read();
if(b[0] == 'G' && b[1] == 'I' && b[2] == 'F' && b[3] == '8' &&
(b[4] == '7' || b[4] == '9') && b[5] == 'a'){
fileType ="GIF";
}else if((b[0] == 0x42) && (b[1] == 0x4d)){
fileType="BMP";
}else if((byte1 == 0xFF) && (byte2 == JPEG.SOI)){
fileType="JPG";
}else if((b[0] == (byte)137 &&
b[1] == (byte)80 &&
b[2] == (byte)78 &&
b[3] == (byte)71 &&
b[4] == (byte)13 &&
b[5] == (byte)10 &&
b[6] == (byte)26 &&
b[7] == (byte)10)){
fileType ="PNG";
}else if(((b[0] == (byte)0) &&
b[1] == 0 &&
((b[2] & 0x8f) != 0 || (b[2] & 0x7f) != 0))){
fileType = "wbmp";
}
} catch (Exception e) {
e.printStackTrace();
}
return fileType;
}
注：以上代码获得的“图片类型”并不是图片的后缀名信息，是图片的实际类型信息。
写到这里实际只是在做JDK源码分析，在实际应用中我们都已经获得了ImageInputStream，恐怕没人会闲着没事的重写JDK的提供好的方法 reader.getFormatName()获得图片类型的方法。
下面说一下我目前所应用场景，了解Spring MVC的就会对其中的MultipartFile感到熟悉，它是在Spring MVC中默认接受图片上传流的参数，在MultipartFile.getBytes() 方法获取的byte[]其实和ImageInputStream 所得到的byte[]是完全相同的，所以我们就完全可以使用MultipartFile.getBytes() 直接进行数组读取判断获取图片类型，代码如下所示：
private String readPictureType(MultipartFile file) throws IOException {
byte[] buffer = Arrays.copyOf(file.getBytes(), 8);
String fileType="JPG";
if(buffer[0] == 'G' && buffer[1] == 'I' && buffer[2] == 'F' && buffer[3] == '8' &&
(buffer[4] == '7' || buffer[4] == '9') && buffer[5] == 'a'){
fileType ="GIF";
}else if((buffer[0] == 0x42) && (buffer[1] == 0x4d)){
fileType="BMP";
}else if((buffer[0] == (byte)137 &&
buffer[1] == (byte)80 &&
buffer[2] == (byte)78 &&
buffer[3] == (byte)71 &&
buffer[4] == (byte)13 &&
buffer[5] == (byte)10 &&
buffer[6] == (byte)26 &&
buffer[7] == (byte)10)){
fileType ="PNG";
}else{
fileType="JPG";
}
return fileType;
}
从而避免繁琐的IO流的操作，节省效率。