從前幾天打開了買了1個半月的書《spring原始碼深度解析》（為自己找個冠冕堂皇的介面，忙工作沒時間看。。。。咳、咳！），在電腦上拉下了spring原始碼，檢視前幾頁的介紹資源載入時候的一些類和介面，終於配好了環境，隨便看下原始碼，印象最深的應該是Resource,畢竟書的開篇前幾頁就是介紹的這個。因為拉的最新版的spring看到了jdk1.8的特性在當中的使用，看到了介面中寫預設方法的特性了，下面是Resource的原始碼：

public interface Resource extends InputStreamSource {

   /**
    * Determine whether this resource actually exists in physical form.
 
    * <p>This method performs a definitive existence check, whereas the
    * existence of a {@code Resource} handle only guarantees a valid
    * descriptor handle.
    */
boolean exists();
/**
    * Indicate whether the contents of this resource can be read via
    * {@link #getInputStream()}.
 
    * <p>Will be {@code true} for typical resource descriptors;
    * note that actual content reading may still fail when attempted.
    * However, a value of {@code false} is a definitive indication
    * that the resource content cannot be read.
    * @see #getInputStream()
    */
default boolean 
 
isReadable() {
      return true;
}

   /**
    * Indicate whether this resource represents a handle with an open stream.
    * If {@code true}, the InputStream cannot be read multiple times,
    * and must be read and closed to avoid resource leaks.
    * <p>Will be {@code false} for typical resource descriptors.
    */
default boolean isOpen() {
      return false;
}

   /**
    * Determine whether this resource represents a file in a file system.
    * A value of {@code true} strongly suggests (but does not guarantee)
    * that a {@link #getFile()} call will succeed.
    * <p>This is conservatively {@code false} by default.
    * @since 5.0
    * @see #getFile()
    */
default boolean isFile() {
      return false;
}

   /**
    * Return a URL handle for this resource.
    * @throws IOException if the resource cannot be resolved as URL,
    * i.e. if the resource is not available as descriptor
    */
URL getURL() throws IOException;
/**
    * Return a URI handle for this resource.
    * @throws IOException if the resource cannot be resolved as URI,
    * i.e. if the resource is not available as descriptor
    * @since 2.5
    */
URI getURI() throws IOException;
/**
    * Return a File handle for this resource.
    * @throws java.io.FileNotFoundException if the resource cannot be resolved as
    * absolute file path, i.e. if the resource is not available in a file system
    * @throws IOException in case of general resolution/reading failures
    * @see #getInputStream()
    */
File getFile() throws IOException;
/**
    * Return a {@link ReadableByteChannel}.
    * <p>It is expected that each call creates a <i>fresh</i> channel.
    * <p>The default implementation returns {@link Channels#newChannel(InputStream)}
    * with the result of {@link #getInputStream()}.
    * @return the byte channel for the underlying resource (must not be {@code null})
    * @throws java.io.FileNotFoundException if the underlying resource doesn't exist
    * @throws IOException if the content channel could not be opened
    * @since 5.0
    * @see #getInputStream()
    */
default ReadableByteChannel readableChannel() throws IOException {
      return Channels.newChannel(getInputStream());
}

   /**
    * Determine the content length for this resource.
    * @throws IOException if the resource cannot be resolved
    * (in the file system or as some other known physical resource type)
    */
long contentLength() throws IOException;
/**
    * Determine the last-modified timestamp for this resource.
    * @throws IOException if the resource cannot be resolved
    * (in the file system or as some other known physical resource type)
    */
long lastModified() throws IOException;
/**
    * Create a resource relative to this resource.
    * @param relativePath the relative path (relative to this resource)
    * @return the resource handle for the relative resource
    * @throws IOException if the relative resource cannot be determined
    */
Resource createRelative(String relativePath) throws IOException;
/**
    * Determine a filename for this resource, i.e. typically the last
    * part of the path: for example, "myfile.txt".
    * <p>Returns {@code null} if this type of resource does not
    * have a filename.
    */
@Nullable
String getFilename();
/**
    * Return a description for this resource,
    * to be used for error output when working with the resource.
    * <p>Implementations are also encouraged to return this value
    * from their {@code toString} method.
    * @see Object#toString()
    */
String getDescription();
}

        其中的isReadable()，isOpen()，isFile()，readableChannel()，都改使用了default方法，

        於是自己去寫了下幾個預設方法測試了一下：測試如下：

測試介面

public interface TestInterface {

    default void methods(){
        System.out.println("介面預設方法");
}
}

測試介面實現類

public class TestDefault implements TestInterface {
    
}

測試：

public class TestMain {

    public static void main(String[] args) {
        TestDefault testDefault = new TestDefault();
testDefault.methods();
}
}

輸出：

我們使用了預設方法之後實現類實現介面的時候可以不用重寫方法，不寫的時候呼叫的是介面的預設方法，

實現類重寫之後：

public class TestDefault implements TestInterface {

    @Override
public void methods() {
        System.out.println("實現類方法");
}
}

測試程式碼一樣，輸出結果如下：

是不是感覺和繼承的時候重寫一樣，那在重寫之後怎樣再去呼叫介面的預設方法呢，這個時候和繼承不太一樣，

程式碼如下：

public class TestDefault implements TestInterface {

    @Override
public void methods() {
        TestInterface.super.methods();
System.out.println("實現類方法");
}
}

測試程式碼一樣、輸出如下：

預設方法的出現可以減少大家在使用介面的時候，減少有多個實現類的時候的程式碼量，把多個實現類的公共程式碼提為預設方法寫在介面中，可以減少程式碼的重複量，