《effective java》之十：序列化

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第74条：谨慎地实现Serializable接口：

 

第75条：考虑使用自定义的序列化形式：

 

public final class StringList implements Serializable {
	private transient int size = 0;
	private transient Entry head = null;

	// No longer Serializable!
	private static class Entry {
		String data;
		Entry next;
		Entry previous;
	}

	// Appends the specified string to the list
	public final void add(String s) {
		// Implementation omitted
	}

	/**
	 * Serialize this {@code StringList} instance.
	 * 
	 * @serialData The size of the list (the number of strings it contains) is
	 *             emitted ({@code int}), followed by all of its elements (each
	 *             a {@code String}), in the proper sequence.
	 */
	private void writeObject(ObjectOutputStream s) throws IOException {
		s.defaultWriteObject();
		s.writeInt(size);

		// Write out all elements in the proper order.
		for (Entry e = head; e != null; e = e.next)
			s.writeObject(e.data);
	}

	private void readObject(ObjectInputStream s) throws IOException,
			ClassNotFoundException {
		s.defaultReadObject();
		int numElements = s.readInt();

		// Read in all elements and insert them in list
		for (int i = 0; i < numElements; i++)
			add((String) s.readObject());
	}

	private static final long serialVersionUID = 93248094385L;
	// Remainder omitted
}

 

第76条：保护性地编写readObject方法：

public final class Period implements Serializable {
    private Date start;
    private Date end;

    /**
     * @param start the beginning of the period
     * @param end   the end of the period; must not precede start
     * @throws IllegalArgumentException if start is after end
     * @throws NullPointerException     if start or end is null
     */
    public Period(Date start, Date end) {
        this.start = new Date(start.getTime());
        this.end = new Date(end.getTime());
        if (this.start.compareTo(this.end) > 0)
            throw new IllegalArgumentException(start + " after " + end);
    }

    public Date start() {
        return new Date(start.getTime());
    }

    public Date end() {
        return new Date(end.getTime());
    }

    public String toString() {
        return start + " - " + end;
    }

    //	readObject method with defensive copying and validity checking - Page 306
    //	This will defend against BogusPeriod and MutablePeriod attacks.
    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();

        // Defensively copy our mutable components
        start = new Date(start.getTime());
        end = new Date(end.getTime());

        // Check that our invariants are satisfied
        if (start.compareTo(end) > 0)
            throw new InvalidObjectException(start + " after " + end);
    }
}

 

第77条：对于实例控制，枚举类型优先于readResolve

还是那句话，单例模式用enum实现最好。

 

第78条：考虑用序列化代理代替序列化实例，序列化最佳实践：

首先，为可序列化类设计一个私有的静态嵌套类，精确表示外围类的实例的逻辑状态，被称作序列化代理。它有一个单独构造器，参数类型就是外围类。外围类和序列代理都实现Serializable接口。

其次，外围类增加writeReplace方法，返回序列代理对象

再次，外围类增加readObject方法，直接抛异常，防止攻击

最后，序列代理类提供readResolve方法，返回逻辑上相当的外围类实例。

 

public final class Period implements Serializable {
	private final Date start;
	private final Date end;

	/**
	 * @param start
	 *            the beginning of the period
	 * @param end
	 *            the end of the period; must not precede start
	 * @throws IllegalArgumentException
	 *             if start is after end
	 * @throws NullPointerException
	 *             if start or end is null
	 */
	public Period(Date start, Date end) {
		this.start = new Date(start.getTime());
		this.end = new Date(end.getTime());
		if (this.start.compareTo(this.end) > 0)
			throw new IllegalArgumentException(start + " after " + end);
	}

	public Date start() {
		return new Date(start.getTime());
	}

	public Date end() {
		return new Date(end.getTime());
	}

	public String toString() {
		return start + " - " + end;
	}

	// Serialization proxy for Period class - page 312
	private static class SerializationProxy implements Serializable {
		private final Date start;
		private final Date end;

		SerializationProxy(Period p) {
			this.start = p.start;
			this.end = p.end;
		}

		private static final long serialVersionUID = 234098243823485285L; // Any
		// readResolve method for Period.SerializationProxy - Page 313
		private Object readResolve() {
			return new Period(start, end); // Uses public constructor
		}
	}

	// writeReplace method for the serialization proxy pattern - page 312
	private Object writeReplace() {
		return new SerializationProxy(this);
	}

	// readObject method for the serialization proxy pattern - Page 313
	private void readObject(ObjectInputStream stream)
			throws InvalidObjectException {
		throw new InvalidObjectException("Proxy required");
	}
}

 

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