Apache Tika源码研究（六）

上文还没有来得及分析Apache Tika是怎样检测文档的mime类型的，以及怎样根据mime类型找到相应的Parser解析类的，下面接着说

在tika-parsers.jar路径文件META-INF/services/org.apache.tika.detect.Detector记录了tika提供的mime类型检测类，当然tika还有部分mime类型检测类该文件并没有记录，后面我通过分析源码可以获知。

该文件包含的检测类我们先睹为快：

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org.apache.tika.parser.microsoft.POIFSContainerDetector
org.apache.tika.parser.pkg.ZipContainerDetector

注意还有vorbis-java-tika-X.jar的同名路径下也存在该文件，tika都会加载进来，所以共包含了三个实现类

org.apache.tika.parser.microsoft.POIFSContainerDetector

org.apache.tika.parser.pkg.ZipContainerDetector

org.gagravarr.tika.OggDetector

这些tika文档mime类型检测类共同实现了Detector接口：

最重要的文件的mime类型检测相关接口和类的UML图如下：

Detector接口源码：

/**
 * Content type detector. Implementations of this interface use various
 * heuristics to detect the content type of a document based on given
 * input metadata or the first few bytes of the document stream.
 *
 * @since Apache Tika 0.3
 */
public interface Detector extends Serializable {

    /**
     * Detects the content type of the given input document. Returns
     * <code>application/octet-stream</code> if the type of the document
     * can not be detected.
     * <p>
     * If the document input stream is not available, then the first
     * argument may be <code>null</code>. Otherwise the detector may
     * read bytes from the start of the stream to help in type detection.
     * The given stream is guaranteed to support the
     * {@link InputStream#markSupported() mark feature} and the detector
     * is expected to {@link InputStream#mark(int) mark} the stream before
     * reading any bytes from it, and to {@link InputStream#reset() reset}
     * the stream before returning. The stream must not be closed by the
     * detector.
     * <p>
     * The given input metadata is only read, not modified, by the detector.
     *
     * @param input document input stream, or <code>null</code>
     * @param metadata input metadata for the document
     * @return detected media type, or <code>application/octet-stream</code>
     * @throws IOException if the document input stream could not be read
     */
    MediaType detect(InputStream input, Metadata metadata) throws IOException;

}

实现该接口的最重要的类是CompositeDetector，该类并不提供具体的mime类型检测，而是调用其他的实现类进行mime类型检测，供tika其他类调用

/**
 * Content type detector that combines multiple different detection mechanisms.
 */
public class CompositeDetector implements Detector {

    /**
     * Serial version UID
     */
    private static final long serialVersionUID = 5980683158436430252L;

    private final MediaTypeRegistry registry;

    private final List<Detector> detectors;

    public CompositeDetector(
            MediaTypeRegistry registry, List<Detector> detectors) {
        this.registry = registry;
        this.detectors = detectors;
    }

    public CompositeDetector(List<Detector> detectors) {
        this(new MediaTypeRegistry(), detectors);
    }

    public CompositeDetector(Detector... detectors) {
        this(Arrays.asList(detectors));
    }

    public MediaType detect(InputStream input, Metadata metadata)
            throws IOException { 
        MediaType type = MediaType.OCTET_STREAM;
        for (Detector detector : getDetectors()) {
            MediaType detected = detector.detect(input, metadata);
            if (registry.isSpecializationOf(detected, type)) {
                type = detected;
            }
        }
        return type;
    }

    /**
     * Returns the component detectors.
     */
    public List<Detector> getDetectors() {
       return Collections.unmodifiableList(detectors);
    }
}

构造函数CompositeDetector(MediaTypeRegistry registry, List<Detector> detectors)用于初始化成员变量MediaTypeRegistry registry和List<Detector> detectors

MediaTypeRegistry registry成员注册了系统提供的mime类型，List<Detector> detectors成员为系统的Detector实现类集合

MediaType detect(InputStream input, Metadata metadata)方法遍历Detector集合检测InputStream input的mime类型

CompositeDetector还有一个派生类DefaultDetector，用于初始化CompositeDetector的成员变量

public class DefaultDetector extends CompositeDetector {

    /** Serial version UID */
    private static final long serialVersionUID = -8170114575326908027L;

    /**
     * Finds all statically loadable detectors and sort the list by name,
     * rather than discovery order. Detectors are used in the given order,
     * so put the Tika parsers last so that non-Tika (user supplied)
     * parsers can take precedence.
     *
     * @param loader service loader
     * @return ordered list of statically loadable detectors
     */
    private static List<Detector> getDefaultDetectors(
            MimeTypes types, ServiceLoader loader) {
        List<Detector> detectors =
                loader.loadStaticServiceProviders(Detector.class);
        Collections.sort(detectors, new Comparator<Detector>() {
            public int compare(Detector d1, Detector d2) {
                String n1 = d1.getClass().getName();
                String n2 = d2.getClass().getName();
                boolean t1 = n1.startsWith("org.apache.tika.");
                boolean t2 = n2.startsWith("org.apache.tika.");
                if (t1 == t2) {
                    return n1.compareTo(n2);
                } else if (t1) {
                    return 1;
                } else {
                    return -1;
                }
            }
        });
        // Finally the Tika MimeTypes as a fallback
        detectors.add(types);
        return detectors;
    }

    private transient final ServiceLoader loader;

    public DefaultDetector(MimeTypes types, ServiceLoader loader) {
        super(types.getMediaTypeRegistry(), getDefaultDetectors(types, loader));
        this.loader = loader;
    }

    public DefaultDetector(MimeTypes types, ClassLoader loader) {
        this(types, new ServiceLoader(loader));
    }

    public DefaultDetector(ClassLoader loader) {
        this(MimeTypes.getDefaultMimeTypes(), loader);
    }

    public DefaultDetector(MimeTypes types) {
        this(types, new ServiceLoader());
    }

    public DefaultDetector() {
        this(MimeTypes.getDefaultMimeTypes());
    }

    @Override
    public List<Detector> getDetectors() {
        if (loader != null) {
            List<Detector> detectors =
                    loader.loadDynamicServiceProviders(Detector.class);
            detectors.addAll(super.getDetectors());
            return detectors;
        } else {
            return super.getDetectors();
        }
    }

}

List<Detector> getDefaultDetectors(MimeTypes types, ServiceLoader loader)方法加载静态的Detector实现类，而List<Detector> getDetectors()方法加载动态的Detector实现类并包含父类的Detector实现类集合
我们这里注意到，前者额外调用了detectors.add(types)，将MimeTypes types对象也添加到集合里面，因为MimeTypes类是实现了Detector接口的，前面文章我已经提到过。
所以实际用到的解析类包括四个

org.apache.tika.parser.microsoft.POIFSContainerDetector

org.apache.tika.parser.pkg.ZipContainerDetector

org.gagravarr.tika.OggDetector

org.apache.tika.mime.MimeTypes

现在我们该如何调用呢，

public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
        ServiceLoader loader = new ServiceLoader();        
        MimeTypes mimeTypes = MimeTypes.getDefaultMimeTypes();        
        Detector detector=new DefaultDetector(mimeTypes, loader);       

        File file=new File("[文件路径]");
        InputStream stream = null;
        try
        {
            stream=new BufferedInputStream(new FileInputStream(file));            
            MediaType type =detector.detect(stream, new Metadata());
            System.out.println("mime类型："+type.toString());
        }
        finally
        {
            if (stream != null)    stream.close();
        }
    }

 现在还有tika怎样加载Parser实现类的，怎样根据文档的mime类型调用相应的Parser实现类的还没有进行分析，不过这些都相对容易分析了，下文再继续吧。