今天读了读HashSet的源码,发现HashSet内部是包装了一个HashMap,构造器是也就是构造内部的HashMap
public HashSet() { map = new HashMap<>(); } public HashSet(Collection<? extends E> c) { map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16)); addAll(c); } public HashSet(int initialCapacity, float loadFactor) { map = new HashMap<>(initialCapacity, loadFactor); } public HashSet(int initialCapacity) { map = new HashMap<>(initialCapacity); } HashSet(int initialCapacity, float loadFactor, boolean dummy) { map = new LinkedHashMap<>(initialCapacity, loadFactor); }
添加删除元素也就是操作内部的HashMap:
public boolean add(E e) { return map.put(e, PRESENT)==null; } public boolean remove(Object o) { return map.remove(o)==PRESENT; } public void clear() { map.clear(); }
value PRESENT是一个final类型的Object对象:
private static final Object PRESENT = new Object();
而内部用来存储数据的field hashMap是一个transient类型的:
private transient HashMap<E,Object> map;
而transient作用就是在序列化对象时候忽略这个属性,既然HashSet的数据都是存放在其内部属性HashMap中的,序列化HashSet忽略它内部的HashMap不就会丢失数据吗?带着这个疑问继续读HashSet的代码,可以发现内部还有两个方法:
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { // Write out any hidden serialization magic s.defaultWriteObject(); // Write out HashMap capacity and load factor s.writeInt(map.capacity()); s.writeFloat(map.loadFactor()); // Write out size s.writeInt(map.size()); // Write out all elements in the proper order. for (E e : map.keySet()) s.writeObject(e); } private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { // Read in any hidden serialization magic s.defaultReadObject(); // Read capacity and verify non-negative. int capacity = s.readInt(); if (capacity < 0) { throw new InvalidObjectException("Illegal capacity: " + capacity); } // Read load factor and verify positive and non NaN. float loadFactor = s.readFloat(); if (loadFactor <= 0 || Float.isNaN(loadFactor)) { throw new InvalidObjectException("Illegal load factor: " + loadFactor); } // Read size and verify non-negative. int size = s.readInt(); if (size < 0) { throw new InvalidObjectException("Illegal size: " + size); } // Set the capacity according to the size and load factor ensuring that // the HashMap is at least 25% full but clamping to maximum capacity. capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f), HashMap.MAXIMUM_CAPACITY); // Constructing the backing map will lazily create an array when the first element is // added, so check it before construction. Call HashMap.tableSizeFor to compute the // actual allocation size. Check Map.Entry[].class since it's the nearest public type to // what is actually created. SharedSecrets.getJavaOISAccess() .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity)); // Create backing HashMap map = (((HashSet<?>)this) instanceof LinkedHashSet ? new LinkedHashMap<E,Object>(capacity, loadFactor) : new HashMap<E,Object>(capacity, loadFactor)); // Read in all elements in the proper order. for (int i=0; i<size; i++) { @SuppressWarnings("unchecked") E e = (E) s.readObject(); map.put(e, PRESENT); } }
大致明白了,在序列化HashSet的时候,会调HashSet中的writeObject,将hashSet中的hashMap中的数据序列化存起来,而在反序列化的时候,会调用Hashset中的readObject来重新构造hashSet中的hashMap,这样的话,就完全没必要序列化hashSet中的HashMap啦,而事实也是如此,debug一下ObjectOutputStream:
private void writeSerialData(Object obj, ObjectStreamClass desc) throws IOException { ObjectStreamClass.ClassDataSlot[] slots = desc.getClassDataLayout(); for (int i = 0; i < slots.length; i++) { ObjectStreamClass slotDesc = slots[i].desc; if (slotDesc.hasWriteObjectMethod()) { PutFieldImpl oldPut = curPut; curPut = null; SerialCallbackContext oldContext = curContext; if (extendedDebugInfo) { debugInfoStack.push( "custom writeObject data (class \"" + slotDesc.getName() + "\")"); } try { curContext = new SerialCallbackContext(obj, slotDesc); bout.setBlockDataMode(true); slotDesc.invokeWriteObject(obj, this); bout.setBlockDataMode(false); bout.writeByte(TC_ENDBLOCKDATA); } finally { curContext.setUsed(); curContext = oldContext; if (extendedDebugInfo) { debugInfoStack.pop(); } } curPut = oldPut; } else { defaultWriteFields(obj, slotDesc); } } }
进 slotDesc.invokeWriteObject(obj, this):
void invokeWriteObject(Object obj, ObjectOutputStream out) throws IOException, UnsupportedOperationException { requireInitialized(); if (writeObjectMethod != null) { try { writeObjectMethod.invoke(obj, new Object[]{ out }); } catch (InvocationTargetException ex) { Throwable th = ex.getTargetException(); if (th instanceof IOException) { throw (IOException) th; } else { throwMiscException(th); } } catch (IllegalAccessException ex) { // should not occur, as access checks have been suppressed throw new InternalError(ex); } } else { throw new UnsupportedOperationException(); } }
writeObjectMethod也就是拿到HashSet中名字为writeObject的method对象:
writeObjectMethod = getPrivateMethod(cl, "writeObject", new Class<?>[] { ObjectOutputStream.class }, Void.TYPE); readObjectMethod = getPrivateMethod(cl, "readObject", new Class<?>[] { ObjectInputStream.class }, Void.TYPE);
再进:writeObjectMethod.invoke(obj, new Object[]{ out });
public Object invoke(Object obj, Object... args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException { if (!override) { if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) { Class<?> caller = Reflection.getCallerClass(); checkAccess(caller, clazz, obj, modifiers); } } MethodAccessor ma = methodAccessor; // read volatile if (ma == null) { ma = acquireMethodAccessor(); } return ma.invoke(obj, args); }
很明显了,就是在反射调用HashSet的writeObject方法,同理反序列化也就是在反射调用HashSet中的readObjcet方法,不在赘述
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