Java同步方法未锁定对象

我编写了一个Java程序来为文件中的每一行生成唯一的ID。可能有多个运行相同程序的作业获得唯一的ID。该ID对于任何文件中的每一行都必须是唯一的。我试图通过将值序列化到文件来实现这一点。由于多个作业将使用该程序，因此我使该方法同步。代码对单个作业正常运行，但是当尝试同时运行多个作业时，同步方法将失败。好像已经有一个对象正在运行时，在同一个对象上再次调用了方法。为什么同步方法不锁定对象？

以下是我编写的ID生成代码–

public class CreateId2 implements Serializable {

private static final long serialVersionUID = 1L;
private long i;

public synchronized long idGenerate() {

    long k = this.generateId(this);
    return k;

}
private long generateId(CreateId2 id){

    long returnedVal = id.getI();
    try {
    returnedVal = id.readObject();
    //System.out.println("previous modiefiied returnedVal is" + returnedVal);
    } catch (FileNotFoundException  ex) {
        id.setI(200L); //start from 200 for the first time
        returnedVal=id.getI();
    } catch (Exception ex2) {
        ex2.printStackTrace();
    }
    returnedVal = returnedVal +1;
    id.setI(returnedVal);
    //System.out.println("returnedVal is " + returnedVal);
    try {
    id.saveObject(id);
    } catch (Exception ex3) {
        ex3.printStackTrace();

        }
    return returnedVal;
}

public void saveObject(CreateId2 id) throws IOException{
    //System.out.println("saving object " + id.getI());
    File savedFileName = new File("C:\\Users\\sam\\Desktop\\tests\\test.ser");
    FileOutputStream savedOutput = new FileOutputStream(savedFileName);
    ObjectOutputStream oos = new ObjectOutputStream(savedOutput);

    oos.writeObject(id);
    oos.flush();
    // close the writing.
    oos.close();
    //System.out.println("saved. value is " + id.getI());
}

public long readObject() throws IOException, ClassNotFoundException{
    //System.out.println("read object is called. value is " + ci2.getI());

    File savedFileName = new File("C:\\Users\\sam\\Desktop\\tests\\test.ser");
    FileInputStream savedInput = new FileInputStream(savedFileName);
    ObjectInputStream ois = new ObjectInputStream(savedInput);

    //long val = Long.parseLong(ois.readObject().toString());

    CreateId2 ci2 = (CreateId2) ois.readObject();
    //System.out.println("value of valtemp " + ci2.getI());
    long val = Long.valueOf(ci2.getI());

    ois.close();
    return val;
    // close the writing.

}

public  long getI() {
    return i;
}

public void setI(long i) {
    this.i = i;
    //CreateId2.i = i;
}

// setter methods

private static CreateId2 instance = null;
private CreateId2() { }
public static CreateId2 getInstance() {
   if (instance == null) {
       instance = new CreateId2();
   }
   return instance;
}

}

我这样称呼这种方法-

CreateId2 id = CreateId2.getInstance();
long j = id.idGenerate();

当我同时调用此方法时，出现以下错误。另外，生成的ID不再是唯一的。

java.io.EOFException
    at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2325)
    at java.io.ObjectInputStream$BlockDataInputStream.readUnsignedShort(ObjectInputStream.java:2806)
    at java.io.ObjectInputStream$BlockDataInputStream.readUTF(ObjectInputStream.java:2864)
    at java.io.ObjectInputStream.readUTF(ObjectInputStream.java:1072)
    at java.io.ObjectStreamClass.readNonProxy(ObjectStreamClass.java:704)
    at java.io.ObjectInputStream.readClassDescriptor(ObjectInputStream.java:830)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1601)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1517)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1771)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1350)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
    at helperPackage.CreateId2.readObject(CreateId2.java:66)
    at helperPackage.CreateId2.generateId(CreateId2.java:23)
    at textFile.RandomTest.main(RandomTest.java:18)