如何附加到现有的java.io.ObjectStream？

Question

至于现在,java.io.StreamCorruptedException当我尝试追加一个Object时,我会得到.我已经在互联网上搜索了一种克服这种情况的方法.到目前为止我找到的答案是无法完成的.解决此问题的方法是将对象写入列表,然后将列表写入文件.

但是每次添加新对象时我都要覆盖该文件.它似乎不是加班的最佳解决方案.

有没有办法将对象附加到现有的对象流？

Answer 1

这实际上很容易.当您要添加到现有流时,您需要使用覆盖的ObjectOutStream的子类,writeStreamHeader以便不在文件中间写入第二个标头.例如

class NoHeaderObjectOutputStream extends ObjectOutputStream {
  public NoHeaderObjectOutputStream(OutputStream os) {
    super(os);
  }
  protected void writeStreamHeader() {}
}

然后只需使用标准的ObjectInputStream来读取整个文件.

Answer 2

我在这个主题上找到的最好的文章是:http: //codify.flansite.com/2009/11/java-serialization-appending-objects-to-an-existing-file/

覆盖ObjectOutputStream的"解决方案"完全错误.我刚刚调查了一个由此引起的错误(浪费了两个宝贵的日子).它不仅有时会损坏序列化文件,而且甚至设法读取而不抛出异常并最终提供垃圾数据(混合字段).对于那些难以置信的人,我附上了一些暴露问题的代码:

import java.io.*;
import java.util.HashMap;
import java.util.Map;

public class Main {

    public static void main(String[] args) throws Exception {

        File storageFile = new File("test");
        storageFile.delete();

        write(storageFile, getO1());
        write(storageFile, getO2());
        write(storageFile, getO2());

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(storageFile));
        read(ois, getO1());
        read(ois, getO2());
        read(ois, getO2());
    }

    private static void write(File storageFile, Map<String, String> o) throws IOException {
        ObjectOutputStream oos = getOOS(storageFile);
        oos.writeObject(o);
        oos.close();
    }

    private static void read(ObjectInputStream ois, Map<String, String> expected) throws ClassNotFoundException, IOException {
        Object actual = ois.readObject();
        assertEquals(expected, actual);
    }

    private static void assertEquals(Object o1, Object o2) {
        if (!o1.equals(o2)) {
            throw new AssertionError("\n expected: " + o1 + "\n actual:   " + o2);
        }
    }

    private static Map<String, String> getO1() {
        Map<String, String> nvps = new HashMap<String, String>();
        nvps.put("timestamp", "1326382770000");
        nvps.put("length", "246");
        return nvps;
    }

    private static Map<String, String> getO2() {
        Map<String, String> nvps = new HashMap<String, String>();
        nvps.put("timestamp", "0");
        nvps.put("length", "0");
        return nvps;
    }

    private static ObjectOutputStream getOOS(File storageFile) throws IOException {
        if (storageFile.exists()) {
            // this is a workaround so that we can append objects to an existing file
            return new AppendableObjectOutputStream(new FileOutputStream(storageFile, true));
        } else {
            return new ObjectOutputStream(new FileOutputStream(storageFile));
        }
    }

    private static class AppendableObjectOutputStream extends ObjectOutputStream {

        public AppendableObjectOutputStream(OutputStream out) throws IOException {
            super(out);
        }

        @Override
        protected void writeStreamHeader() throws IOException {
            // do not write a header
        }
    }
}

如该文章所述,您可以使用以下解决方案之一:

解决方案#1:在单个流中伪造多个文件

...

将您的"事务"写入ByteArrayOutputStream,然后通过DataOutputStream将此ByteArrayOutputStream的长度和内容写入文件.

解决方案#2:重新打开并跳过

另一种解决方案涉及使用以下方

long pos = fis.getChannel().position();

关闭文件,重新打开文件,然后在读取下一个事务之前跳到此位置.