java marker interface

johnhuster的分享

发布于 2022-03-28 19:50:26

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marker interface是一种特殊的接口，这种接口没有任何属性或者方法，java开发者最常用的marker interface应该当属Serializable了，当一个实体类需要通过网络传输时需要确保该类可以被序列化，这样接收端就可以通过反序列化过程还原该类实例。这种接口存在的意义在于其对java编译器的作用，marker interface告知java编译器实现marker interface的类不是一般的类，需要被特殊对待。

下面看下Serializable这个接口如何起作用的，ObjectOutputStream::writeObject0方法

    private void writeObject0(Object obj, boolean unshared)
         throws IOException
     {
         boolean oldMode = bout.setBlockDataMode(false);
         depth++;
         try {
             // handle previously written and non-replaceable objects
             int h;
             if ((obj = subs.lookup(obj)) == null) {
                 writeNull();
                 return;
             } else if (!unshared && (h = handles.lookup(obj)) != -1) {
                 writeHandle(h);
                 return;
             } else if (obj instanceof Class) {
                 writeClass((Class) obj, unshared);
                 return;
             } else if (obj instanceof ObjectStreamClass) {
                 writeClassDesc((ObjectStreamClass) obj, unshared);
                 return;
             }
            // check for replacement object
             Object orig = obj;
             Class<?> cl = obj.getClass();
             ObjectStreamClass desc;
             for (;;) {
                 // REMIND: skip this check for strings/arrays?
                 Class<?> repCl;
                 desc = ObjectStreamClass.lookup(cl, true);
                 if (!desc.hasWriteReplaceMethod() ||
                     (obj = desc.invokeWriteReplace(obj)) == null ||
                     (repCl = obj.getClass()) == cl)
                 {
                     break;
                 }
                 cl = repCl;
             }
             if (enableReplace) {
                 Object rep = replaceObject(obj);
                 if (rep != obj && rep != null) {
                     cl = rep.getClass();
                     desc = ObjectStreamClass.lookup(cl, true);
                 }
                 obj = rep;
             }
            // if object replaced, run through original checks a second time
             if (obj != orig) {
                 subs.assign(orig, obj);
                 if (obj == null) {
                     writeNull();
                     return;
                 } else if (!unshared && (h = handles.lookup(obj)) != -1) {
                     writeHandle(h);
                     return;
                 } else if (obj instanceof Class) {
                     writeClass((Class) obj, unshared);
                     return;
                 } else if (obj instanceof ObjectStreamClass) {
                     writeClassDesc((ObjectStreamClass) obj, unshared);
                     return;
                 }
             }
            // remaining cases
             if (obj instanceof String) {
                 writeString((String) obj, unshared);
             } else if (cl.isArray()) {
                 writeArray(obj, desc, unshared);
             } else if (obj instanceof Enum) {
                 writeEnum((Enum<?>) obj, desc, unshared);
             } else if (obj instanceof Serializable) {
                 writeOrdinaryObject(obj, desc, unshared);
             } else {
                 if (extendedDebugInfo) {
                     throw new NotSerializableException(
                         cl.getName() + "\n" + debugInfoStack.toString());
                 } else {
                     throw new NotSerializableException(cl.getName());
                 }
             }
         } finally {
             depth--;
             bout.setBlockDataMode(oldMode);
         }
     }

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原始发表：2019/01/23 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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