如何复制 java.util.Properties 对象？

Question

我有以下字段和构造函数：

private final Properties properties;

public PropertiesExpander(Properties properties) {
    this.properties = properties;
}

好的做法是在构造函数中复制每个可变集合。我想做一个浅的、独立的副本。我怎样才能做到这一点？

我的第一个想法是使用 putAll() 方法：

private final Properties properties = new Properties();

public PropertiesExpander(Properties properties) {
    this.properties.putAll(properties);
}

有没有更简单、更高效或更惯用的方法来做到这一点？也许在 Guava 或 Apache Commons 中有一些实用程序？

Answer 1

使用 putAll() 很棒...如果您需要继续使用 Properties。它在 O(number of elements) 中运行并且开销很小。我建议的唯一区别是出于性能原因远离 Properties 除非你需要它，因为它继承自 Hashtable. Also, don't use Properties because it doesn't actually conform to any interface, just Dictionary which is an abstract class; this will limit your options. See: What does it mean to "program to an interface"?

As of the Java 2 platform v1.2, this class was retrofitted to implement the Map interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.

无论你做什么，都不要使用 clone()， 它既不安全也不高效。参见：

您编辑了问题以询问有关 Guava 和 apache-commons 的问题。如果是纯粹的防御副本，而且是不可变的，我会推荐使用Map<String, String> map = ImmutableMap.copyOf(properties). Note: again, this doesn't use an actual Properties object because Hashtable is not recommended unless you need it. From the wiki

When you don't expect to modify a collection, or expect a collection to remain constant, it's a good practice to defensively copy it into an immutable collection.

Important: Each of the Guava immutable collection implementations rejects null values. We did an exhaustive study on Google's internal code base that indicated that null elements were allowed in collections about 5% of the time, and the other 95% of cases were best served by failing fast on nulls. If you need to use null values, consider using Collections.unmodifiableList and its friends on a collection implementation that permits null. More detailed suggestions can be found here.

Answer 2

Properties 实现了 Cloneable，因此您可以根据需要执行以下操作。

this.properties = (Properties) properties.clone();

将此添加到您的 class

protected Object clone() throws CloneNotSupportedException {
    return super.clone();
}

或者，如果您担心使用克隆，您的 class 也实现了可序列化，因此您可以这样做。

import org.apache.commons.lang.SerializationUtils;

this.properties = SerializationUtils.clone(properties);

Properies

Cloneable

Answer 3

或者您可以按 "long" 方式进行：

    Iterator i = properties.keySet().iterator();
    while(i.hasNext()){
        this.properties.put(i.next(), properties.get(i));
    }

迭代器与属性来自同一个 java.util 包，因此没有外部依赖性。

如果编译器关于未检查类型的警告困扰您，您可以简单地将其更改为（假设您的属性键是字符串）：

    Iterator<Object> i = properties.keySet().iterator();
    while(i.hasNext()){
        this.properties.put(i.next().toString(), properties.get(i));
    }

Answer 4

试试这个：

Properties newProps = new Properties();
properties.forEach((key, value) -> {
    newProps.setProperty((String) key, (String) value);
});

Answer 5

下面的程序将一个属性文件复制到另一个文件，并且还会删除重复项。源属性文件可能有重复的属性，但它会创建没有任何重复属性的新文件。

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Properties;
import java.util.Set;

public class App2 {

    public static void main(String[] args) {

        try (InputStream input = new FileInputStream("config.properties")) {

            Properties prop = new Properties();

            // load a properties file
            prop.load(input);

            writeToNewProperties(prop);
        } catch (IOException ex) {
            ex.printStackTrace();
        }

    }

    private static void writeToNewProperties(Properties prop) {
        Properties outPutProp = new Properties();
        // get the property value and print it out
        Set<String> stringPropertyNames = prop.stringPropertyNames();
        for (String propertyName : stringPropertyNames) {
            outPutProp.put(propertyName, prop.get(propertyName));
        }

        try (OutputStream output = new FileOutputStream("out-config.properties")) {
            // save properties to project root folder
            outPutProp.store(output, null);
            System.out.println(outPutProp);
        } catch (IOException io) {
            io.printStackTrace();
        }
    }

}

Answer 6

另一种方法就是

Properties newVersion = new Properties(originalVersion);

它将表现为最初作为originalVersion的副本，除非newVersion中的键被删除，它将恢复为originalVersion 中存储的值。实际上，originalVersion 表现为默认的 key/value 集。

如何复制 java.util.Properties 对象？

How do I make a copy of java.util.Properties object?

java

collections

copy

properties

copy-constructor