Read WeakHashMap source code

//A Map object based on weak references
// first look at the constructor

public WeakHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

public WeakHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

public WeakHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR);
        putAll(m);
    }

public WeakHashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Initial Capacity: "+
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;

        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load factor: "+
                                               loadFactor);
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        table = newTable(capacity);
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
        useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
    }

// add element
 public V put(K key, V value) {
        // Convert the key value of null to an object
        Object k = maskNull(key);
	//Calculate the hash value
        int h = hash(k);
	//Get the entire entry, which will delete the expired key
        Entry<K,V>[] tab = getTable();
        int i = indexFor(h, tab.length);

        for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
	   //if the key already exists
            if (h == e.hash && eq(k, e.get())) {
                V oldValue = e.value;
                if (value != oldValue)
                    e.value = value;
                return oldValue;
            }
        }

        modCount++;
        Entry<K,V> e = tab[i];
	// insert at the first position
        tab[i] = new Entry<>(k, value, queue, h, e);
	//expand
        if (++size >= threshold)
            resize(tab.length * 2);
        return null;
    }

 private static Object maskNull(Object key) {
        return (key == null) ? NULL_KEY : key;
    }
 private static final Object NULL_KEY = new Object();

private Entry<K,V>[] getTable() {
        expungeStaleEntries();
        return table;
    }

// delete the expired key
 private void expungeStaleEntries() {
        for (Object x; (x = queue.poll()) != null; ) {
            synchronized (queue) {
                @SuppressWarnings("unchecked")
                    Entry<K,V> e = (Entry<K,V>) x;
                int i = indexFor(e.hash, table.length);

                Entry<K,V> prev = table[i];
                Entry<K,V> p = prev;
                while (p != null) {
                    Entry<K,V> next = p.next;
                    if (p == e) {
		        // delete the head of the queue at this time
                        if (prev == e)
                            table[i] = next;
                        else
                            prev.next = next;
                        // Must not null out e.next;
                        // stale entries may be in use by a HashIterator
			// prevent memory leaks
                        e.value = null; // Help GC
                        size--;
                        break;
                    }
                    prev = p;
                    p = next;
                }
            }
        }
    }

 void resize(int newCapacity) {
        Entry<K,V>[] oldTable = getTable();
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry<K,V>[] newTable = newTable(newCapacity);
        boolean oldAltHashing = useAltHashing;
        useAltHashing |= sun.misc.VM.isBooted() &&
                (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        boolean rehash = oldAltHashing ^ useAltHashing;
        transfer(oldTable, newTable, rehash);
        table = newTable;

       
        if (size >= threshold / 2) {
            threshold = (int)(newCapacity * loadFactor);
        } else {
            expungeStaleEntries();
            transfer(newTable, oldTable, false);
            table = oldTable;
        }
    }

// Construct a new entry
  private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest, boolean rehash) {
        for (int j = 0; j < src.length; ++j) {
            Entry<K,V> e = src[j];
            src[j] = null;
            while (e != null) {
                Entry<K,V> next = e.next;
                Object key = e.get();
                if (key == null) {
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                } else {
                    if (rehash) {
                        e.hash = hash(key);
                    }
                    int i = indexFor (e.hash, dest.length);
                    e.next = dest[i];
                    dest[i] = e;
                }
                e = next;
            }
        }
    }

/** The process of inserting elements is as above. Now the question is how to recycle automatically? */
//Look at this code when inserting
tab[i] = new Entry<>(k, value, queue, h, e);
//then see this entry
 private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
        V value;
        int hash;
        Entry<K,V> next;

        /**
         * Creates new entry.
         */
        Entry(Object key, V value,
              ReferenceQueue<Object> queue,
              int hash, Entry<K,V> next) {
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

//then just
public abstract class Reference<T>
//There is a thread inside
private static class ReferenceHandler extends Thread {

        ReferenceHandler(ThreadGroup g, String name) {
            super(g, name);
        }

        public void run() {
            for (;;) {

                Reference r;
                synchronized (lock) {
                    if (pending != null) {
		       //assigned by the virtual machine
                        r = pending;
                        Reference rn = r.next;
                        pending = (rn == r) ? null : rn;
                        r.next = r;
                    } else {
                        try {
                            lock.wait();
                        } catch (InterruptedException x) { }
                        continue;
                    }
                }

                // Fast path for cleaners
                if (r instanceof Cleaner) {
                    ((Cleaner)r).clean();
                    continue;
                }

                ReferenceQueue q = r.queue;
		// enqueue
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        }
    }



boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
        synchronized (r) {
            if (r.queue == ENQUEUED) return false;
            synchronized (lock) {
                r.queue = ENQUEUED;
                r.next = (head == null) ? r : head;
                head = r;
                queueLength++;
                if (r instanceof FinalReference) {
                    sun.misc.VM.addFinalRefCount(1);
                }
                lock.notifyAll();
                return true;
            }
        }
    }

/**
Summary: Every time an element is put, it first finds the expired key from the ReferenceQueue and then deletes it.
Then every time the entry is created, the background will manage a group of threads through ThreadGroup. This group of threads is mainly
The jvm assigns an expired key to the Reference. Then add it to the queue of ReferenceQueue.
*/


public void putAll(Map<? extends K, ? extends V> m) {
        int numKeysToBeAdded = m.size();
        if (numKeysToBeAdded == 0)
            return;

        //First calculate whether expansion is needed to improve efficiency
        if (numKeysToBeAdded > threshold) {
            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
            if (targetCapacity > MAXIMUM_CAPACITY)
                targetCapacity = MAXIMUM_CAPACITY;
            int newCapacity = table.length;
            while (newCapacity < targetCapacity)
                newCapacity <<= 1;
            if (newCapacity > table.length)
                resize(newCapacity);
        }

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

// get value by key
public V get(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K,V> e = tab[index];
        while (e != null) {
            if (e.hash == h && eq(k, e.get()))
                return e.value;
            e = e.next;
        }
        return null;
    }

//return length
public int size() {
        if (size == 0)
            return 0;
        expungeStaleEntries();
        return size;
    }

//Whether the entire map is empty
public boolean isEmpty() {
        return size() == 0;
    }

// does it contain a key
 public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }

  Entry<K,V> getEntry(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K,V> e = tab[index];
        while (e != null && !(e.hash == h && eq(k, e.get())))
            e = e.next;
        return e;
    }


// delete by key
public V remove(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int i = indexFor(h, tab.length);
        Entry<K,V> prev = tab[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            if (h == e.hash && eq(k, e.get())) {
                modCount++;
                size--;
                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;
                return e.value;
            }
            prev = e;
            e = next;
        }

        return null;
    }


//delete entry
boolean removeMapping(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Entry<K,V>[] tab = getTable();
        Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
        Object k = maskNull(entry.getKey());
        int h = hash(k);
        int i = indexFor(h, tab.length);
        Entry<K,V> prev = tab[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            if (h == e.hash && e.equals(entry)) {
                modCount++;
                size--;
                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;
                return true;
            }
            prev = e;
            e = next;
        }

        return false;
    }

// clear
public void clear() {
        
        while (queue.poll() != null)
            ;

        modCount++;
        Arrays.fill(table, null);
        size = 0;

        
        while (queue.poll() != null)
            ;
    }

// does it contain a value
public boolean containsValue(Object value) {
        if (value==null)
            return containsNullValue();

        Entry<K,V>[] tab = getTable();
        for (int i = tab.length; i-- > 0;)
            for (Entry<K,V> e = tab[i]; e != null; e = e.next)
                if (value.equals(e.value))
                    return true;
        return false;
    }