如何在 Java Dynamic WebService 应用程序中写入本地文件？

Question

I have a file that contains json data Books.json . The file is located in package com.ebook.database . I can successfully read the file's contents, but I when I try to write to the file, I get a FileNotFoundException .

Below is my project's strudture, showing the package in which the Books.json file is located:

Below is the code I use to successfully read the file's contents ( filePath receives a String value of /com/ebook/database/Books.json and reads the file just fine:

/**
     * A utility method for reading the data stored in a json file.
     * 
     * @param filePath path to the desired database table 
     * @return the contents of the desired table
     * @throws IOException occurs when the desired file is not found
     */
    public static String readFile(String filePath) throws IOException {
        InputStream inputStream = DBHelper.class.getResourceAsStream(filePath);
        String content = IOUtils.toString(inputStream, StandardCharsets.UTF_8);
        return content;
    }

Below is the code I'm using to try to access the same Books.json file for writing, unsuccessfully:

/**
     * Adds a new Book to the Books.json file
     * */
    public void postBook(Book book, String filePath) {
        
        //create Gson instance with pretty-print
        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        
        //First, convert the Product object into a Json string
        try {
               
            File file = new File(filePath);
            OutputStream out = new FileOutputStream(file);
            
            /* This logic will check whether the file
               * exists or not. If the file is not found
               * at the specified location it would create
               * a new file*/
              if (!file.exists()) {
                 file.createNewFile();
              }
            
               List<Book> books;
               String updatedBooksJson;
               
               //if file is empty, just throw the object in -- no prior comma needed.      
               if(DBHelper.tableIsEmpty(filePath)) {
                   books = new ArrayList<Book>();
                   books.add(book);
                   
                   updatedBooksJson = gson.toJson(books);
                   
                   //writing to the file.
                   byte[] bytes = updatedBooksJson.getBytes();
                   out.write(bytes[6]);
                   out.flush();
                    
               } else {
                   //The file is not empty.  Read the list of Products currently
                   //in the file, convert to List<Product>, add the new Product, and
                   //update the file
                   
                   books = getAllBooks(filePath);
                   
                   //add the new product to the existing list of products:
                   books.add(book);
                   
                   //write the updated products list to the Products.json file
                   updatedBooksJson = gson.toJson(books);
                   
                   //clear the old file contents
                   FileChannel.open(Paths.get(filePath), StandardOpenOption.WRITE).truncate(0).close();
                      
                   //write the new file contents
                 //writing to the file.
                   byte[] bytes = updatedBooksJson.getBytes();
                   out.write(bytes[6]);
                   out.flush();
               }
               
               out.close();
            } catch (IOException e) {
                e.printStackTrace();
              }
    }

Below is the resulting stacktrace when attempting to write to the /com/ebook/database/Books.json file:

INFO: Server startup in 2782 ms
java.io.FileNotFoundException: \com\ebook\database\Books.json (The system cannot find the path specified)
    at java.base/java.io.FileOutputStream.open0(Native Method)
    at java.base/java.io.FileOutputStream.open(FileOutputStream.java:292)
    at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:235)
    at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:185)
    at com.ebook.dal.BookDAO.postBook(BookDAO.java:59)
    at com.ebook.services.BookService.addNewBook(BookService.java:38)
    at com.ebook.resources.BookResource.postNewBook(BookResource.java:72)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory.lambda$static$0(ResourceMethodInvocationHandlerFactory.java:52)
    at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:124)
    at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:167)
    at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:219)
    at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:79)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:469)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:391)
    at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:80)
    at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:253)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:248)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:244)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:292)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:274)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:244)
    at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:265)
    at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:232)
    at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:680)
    at org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:392)
    at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:346)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:365)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:318)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:205)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:492)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:165)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:1025)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:452)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1201)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:654)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:317)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.base/java.lang.Thread.run(Thread.java:835)
Oct 11, 2020 3:32:40 PM org.apache.catalina.core.StandardContext reload

Answer 1

You are mixing classpath and local file system paths here. You should never try to overwrite elements in the classpath like that without knowing what you're doing as that would potentially charge the behavior of the app when you restart it. Just put the somewhere on the file system and consistently read and write there.