Java 在我的apache chemistry CMIS中，我在同步两个文档时遇到了一个冲突错误

这是我的CMIS同步代码

    String documentName = null;
    StringBuilder contentBuilder= new StringBuilder();
    ByteArrayInputStream inputStream;
    byte[] buffer = null;

    session=getSession(sourceClientIbmNet);
    CmisObject cmisObjectSource = session.getObject(sourceId);
    if (BaseTypeId.CMIS_DOCUMENT.equals(cmisObjectSource.getBaseTypeId())) {
                Document document = (Document) cmisObjectSource;

        List<String> fileContent = getFileContent(document);

        for(String string: fileContent){
            contentBuilder= contentBuilder.append(string+"\n");
                            }
    }
    session = null;
    session=    getSession(destinatioClientIbmNet);

    CmisObject cmisObjectDestination=session.getObject(destinationId);

    if (BaseTypeId.CMIS_DOCUMENT.equals(cmisObjectDestination.getBaseTypeId())) {

        Document document = (Document) cmisObjectDestination;
        documentName = document.getName();

         if (((DocumentType) (document.getType())).isVersionable()) {
             document.refresh();
        Document pwc = (Document) session.getObject(document.checkOut());//Here i got exception
                String mimetype = "text/plain; UTF-8";
                String content = contentBuilder.toString();
                try {
                    buffer = content.getBytes("UTF-8");
                } catch (Exception e) {
                }

                inputStream = new ByteArrayInputStream(buffer);

                ContentStream contentStream = session.getObjectFactory()
                        .createContentStream(document.getName(), buffer.length,
                                mimetype, inputStream);
                try {
                    pwc.checkIn(true, null, contentStream, "minor version");
                } catch (CmisBaseException e) {
                    e.printStackTrace();
                    pwc.cancelCheckOut();
                }
                if (contentStream != null) {
                    List<String> newContent;
                    ContentStream contentStream11 = document.getContentStream();
                    newContent = getFileContent(document);
                }
                }
        }

org.apache.chemistry.opencmis.commons.exceptions.CmisConstraintException: Conflict
    at org.apache.chemistry.opencmis.client.bindings.spi.atompub.AbstractAtomPubService.convertStatusCode(AbstractAtomPubService.java:499)
    at org.apache.chemistry.opencmis.client.bindings.spi.atompub.AbstractAtomPubService.post(AbstractAtomPubService.java:661)
    at org.apache.chemistry.opencmis.client.bindings.spi.atompub.VersioningServiceImpl.checkOut(VersioningServiceImpl.java:95)
    at org.apache.chemistry.opencmis.client.runtime.DocumentImpl.checkOut(DocumentImpl.java:237)
    at com.aegis.web.util.CmisClientIbmNet.updateContent(CmisClientIbmNet.java:304)
    at com.aegis.web.controller.RepositoryController.sourceToDestinationSYNC(RepositoryController.java:329)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)