java - 如何自动写入和读取文件？

Question

我有一个 java 应用程序有 3 ArrayList对象:items , customers和 orders .

我想知道如何在关闭应用程序时将这些对象写入文件，然后在启动应用程序时如何从文件中读取？

这是我得到的帮助我的代码:

//Write to file
ObjectOutputStream outputStream;
outputStream=new ObjectOutputStream(new FileOutputStream("customerFile",true));
outputStream.writeObject(customers);
outputStream.close();

//Read from file 
Clients fileCust = null;
ObjectInputStream inputStream;
inputStream = new ObjectInputStream(new FileInputStream("customerFile"));
fileCust = (Clients)inputStream.readObject();
inputStream.close();

我想我可以通过单击按钮进行读写来弄清楚如何执行此操作，但我想知道如何自动执行此操作。 (程序开始和结束时)。

Answer 1

要在程序开始读取文件，只需将读取代码放在开始即可。

但是要确保在您从任何地方 退出程序时保存它，您可以使用shutdown hook。 :

Runtime.getRuntime().addShutdownHook(new Thread(() -> {
    // Shutdown code here
}));

您在程序开始时注册的。它在 JVM 关闭时被调用。通过文档:

The Java virtual machine shuts down in response to two kinds of events:

• The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
• The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.

---

现在在你的程序开始时你会做这样的事情:

public static void main(String... args) {
    // getConfig() returns a configuration object read from a file
    // Or a new object if no file was found.
    Configuration config = getConfig();

    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
        ...
        // Note that I'm not appending to, but overwriting the file
        ObjectOutputStream outputStream = 
            new ObjectOutputStream(new FileOutputStream("myConfigFile"));

        outputStream.writeObject(config); // write the config object.
        ...
    }));

    // Rest of the program, using 'config'
}

需要注意的是，因为关闭 Hook 是在程序结束时执行的，所以它会写入配置对象的最终状态。因此，在程序期间对“配置”所做的更改将被考虑在内。

---

为了能够像这样编写对象，它们需要实现 java.io.serializable 并且它们的所有字段也是如此。具有 transient 限定符的字段除外。

配置对象可能看起来像这样:

public class Configuration implements java.io.Serializable {

    // transient -> Will not be written or read.
    // So it is always 'null' at the start of the program.
    private transient Client lastClient = null;

    // 'Client' must also implement java.io.serializable,
    // for 'Configuration' to be serializable.
    // Otherwise, an exception is thrown when writing the object.
    private List<Client> clients = new ArrayList<Client>();

    // 'String' already implements java.io.serializable.
    private String someString;

    ...
}