JDK1.8源码中的设计模式

Posted 2020-07-07 zhongrui_fzr

1. 迭代器与组合模式(Iterator)

1.1 Collection.iterator()

集合（Collection）指的是一群对象，其存储方式可以是各式各样的数据结构。
如何能让客户遍历你的对象而又无法窥视你存储对象的方式——利用迭代器（iterator）java.util.Iterator来封装“遍历集合内的每个对象的过程”。

//忽略集合实现方式（列表、数组、散列表）封装对象遍历
public interface Iterator<E> {
    boolean hasNext();
    E next();
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

1.2 java.util.Enumeration

枚举类型Enumeration提供与iterator类似的功能

public interface Enumeration<E> {
    boolean hasMoreElements();
    E nextElement();
}

2. 模板方法模式(Template Method)

2.1 java.util.AbstractList的所有非抽象方法

以及java.util.AbstractSet,java.util.AbstractMap的所有非抽象方法

package java.util;
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    protected AbstractList() {
    }
    public boolean add(E e) {
        add(size(), e);
        return true;
    }
    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }
     // Search Operations
    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return it.previousIndex();
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }
    public int lastIndexOf(Object o) {
        ListIterator<E> it = listIterator(size());
        if (o==null) {
            while (it.hasPrevious())
                if (it.previous()==null)
                    return it.nextIndex();
        } else {
            while (it.hasPrevious())
                if (o.equals(it.previous()))
                    return it.nextIndex();
        }
        return -1;
    }
    public void clear() {
        removeRange(0, size());
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }
    // Iterators
    public Iterator<E> iterator() {
        return new Itr();
    }
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }
    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);
        return new ListItr(index);
    }
    public List<E> subList(int fromIndex, int toIndex) {
        return (this instanceof RandomAccess ?
                new RandomAccessSubList<>(this, fromIndex, toIndex) :
                new SubList<>(this, fromIndex, toIndex));
    }
    // Comparison and hashing
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;

        ListIterator<E> e1 = listIterator();
        ListIterator<?> e2 = ((List<?>) o).listIterator();
        while (e1.hasNext() && e2.hasNext()) {
            E o1 = e1.next();
            Object o2 = e2.next();
            if (!(o1==null ? o2==null : o1.equals(o2)))
                return false;
        }
        return !(e1.hasNext() || e2.hasNext());
    }
    public int hashCode() {
        int hashCode = 1;
        for (E e : this)
            hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
        return hashCode;
    }
    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }
    protected transient int modCount = 0;

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size();
    }
}

2.2 模板方法模式(Template Method)

Applet.init()/start()/stop()/destroy()/paint()

Applet中的init()/start()/stop()/destroy()/paint()这些方法，都是hook

// Applet  
    public void init() { //什么也不做的hook  
    }

// Beans  
   public static Object instantiate(ClassLoader cls, String beanName,
                    BeanContext beanContext, AppletInitializer initializer)
                        throws IOException, ClassNotFoundException {



                // If it was deserialized then it was already init-ed.  
                // Otherwise we need to initialize it.  

                if (!serialized) {
                    // We need to set a reasonable initial size, as many  
                    // applets are unhappy if they are started without  
                    // having been explicitly sized.  
                    applet.setSize(100,100);
                    applet.init(); //调用hook  
                }


        }

        return result;
    }

2.3 Arrays.sort()

// Arrays
public static void sort(Object[] a) {
    …
    ComparableTimSort.sort(a);
}
// ComparableTimSort
static void sort(Object[] a, int lo, int hi) {
    …
    binarySort(a, lo, hi, lo + initRunLen);
}

//算法框架在工具类中实现
    private static void binarySort(Object[] a, int lo, int hi, int start) {
        assert lo <= start && start <= hi;
        if (start == lo)
            start++;
        for ( ; start < hi; start++) {
            @SuppressWarnings("unchecked")
            Comparable<Object> pivot = (Comparable) a[start];

            // Set left (and right) to the index where a[start] (pivot) belongs  
            ....
            while (left < right) {
                int mid = (left + right) >>> 1;
                if (pivot.compareTo(a[mid]) < 0) //compareTo这个算法步骤，是由各个Comparable的子类定义的  
                    right = mid;
                else
                    left = mid + 1;
            }
            ....
        }
    }

2.4 InputStream.read()

//算法框架  
 public int read(byte b[], int off, int len) throws IOException {
        ...
        int c = read();
        ...
}
//算法步骤由子类实现  
public abstract int read() throws IOException;

3. 工厂方法模式（Factory Method）

3.1 java.lang.Integer#valueOf(String)

Boolean, Byte, Character,Short, Long, Float 和 Double与之类似

public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

3.2 java.lang.Class#forName()

 public static Class<?> forName(String className)
                throws ClassNotFoundException {
        return forName0(className, true,
                        ClassLoader.getClassLoader(Reflection.getCallerClass()));
    }
/** Called after security checks have been made. */
private static native Class<?> forName0(String name, boolean initialize, ClassLoader loader) throws ClassNotFoundException;
    @CallerSensitive
    public T newInstance() throws InstantiationException, IllegalAccessException
    {
        if (System.getSecurityManager() != null) {
            checkMemberAccess(Member.PUBLIC, Reflection.getCallerClass(), false);
        }
        // NOTE: the following code may not be strictly correct under
        // the current Java memory model.
        // Constructor lookup
        if (cachedConstructor == null) {
            if (this == Class.class) {
                throw new IllegalAccessException(
                    "Can not call newInstance() on the Class for java.lang.Class"
                );
            }
            try {
                Class<?>[] empty = {};
                final Constructor<T> c = getConstructor0(empty, Member.DECLARED);
                // Disable accessibility checks on the constructor
                // since we have to do the security check here anyway
                // (the stack depth is wrong for the Constructor‘s
                // security check to work)
                java.security.AccessController.doPrivileged(
                    new java.security.PrivilegedAction<Void>() {
                        public Void run() {
                                c.setAccessible(true);
                                return null;
                            }
                        });
                cachedConstructor = c;
            } catch (NoSuchMethodException e) {
                throw (InstantiationException)
                    new InstantiationException(getName()).initCause(e);
            }
        }
        Constructor<T> tmpConstructor = cachedConstructor;
        // Security check (same as in java.lang.reflect.Constructor)
        int modifiers = tmpConstructor.getModifiers();
        if (!Reflection.quickCheckMemberAccess(this, modifiers)) {
            Class<?> caller = Reflection.getCallerClass();
            if (newInstanceCallerCache != caller) {
                Reflection.ensureMemberAccess(caller, this, null, modifiers);
                newInstanceCallerCache = caller;
            }
        }
        // Run constructor
        try {
            return tmpConstructor.newInstance((Object[])null);
        } catch (InvocationTargetException e) {
            Unsafe.getUnsafe().throwException(e.getTargetException());
            // Not reached
            return null;
        }
    }

4. 适配器模式（Adapter）

4.1 java.util.Arrays#asList()

//Arrays
 public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

4.2 RunnableAdapter

完整类名：java.util.concurrent.Executors.RunnableAdapter<T>
FutureTask接受一个Callable参数，但是如果有的是Runnable该怎么办？
FutureTask本身提供了适配：

/**
 * Creates a <tt>FutureTask</tt> that will upon running, execute the given <tt>Callable</tt>.
 */
public FutureTask(Callable<V> callable) {
    sync = new Sync(callable);
}

/**
 * Creates a <tt>FutureTask</tt> that will upon running, execute the given <tt>Runnable</tt>
 */
public FutureTask(Runnable runnable, V result) {
    sync = new Sync(Executors.callable(runnable, result));
}

Executors.callable()返回Adapter对象：

public static <T> Callable<T> callable(Runnable task, T result) {
    return new RunnableAdapter<T>(task, result);
}

/** --Adapter!--
 * A callable that runs given task and returns given result
 */
static final class RunnableAdapter<T> implements Callable<T> {  //Target
    final Runnable task; //Adaptee
    final T result;
    RunnableAdapter(Runnable  task, T result) {
        this.task = task;
        this.result = result;
    }
    public T call() {
        task.run();
        return result;
    }
}

待补充