1

我有以下单例类

public class TestFileEngine {
    private static int counter = 0;
    private static List<GeneratedFile> generatedFileList;
    private static Optional<TestFileEngine> engine = Optional.empty();

    private TestFileEngine() {
        generatedFileList = Collections.synchronizedList(new ArrayList<>());
    }

    public static synchronized TestFileEngine getInstance() {
        if (!engine.isPresent()) {
            engine = Optional.of(new TestFileEngine());
        }

        return ruEngine.get();
    }

    public synchronized void generateFile() {
        counter++;
        String timestamp = LocalDateTime.now().toString();
        String content = "";//insert random content generation
        GeneratedFile gf = new GeneratedFile(counter + ".txt", timestamp, content);
        generatedFileList.add(gf);
        System.out.println("Generated file " + counter + ".txt");
    }

    public GeneratedFile findFileByName(String filename) {
        for (GeneratedFile file : generatedFileList){
           if(file.getFileName().equals(filename)){
               return file;
           }
        }
        return null;
    }
}

现在我想拥有两个独立的引擎（将来可能更多），用于跟踪目的，我偶然发现了 multiton 模式，仍然使用惰性实现。所以我将更改以下内容：

//Getting rid of Optional, as it can get funky with Maps
private static final Map<String, TestFileEngine> _engines = new HashMap<String, TestFileEngine>();

public static synchronized TestFileEngine getInstance(String key) {
    if (_engines.get(key) == null) {
        _engines.put(key, new TestFileEngine());
        System.out.println("Create engine " + key);
    }else {
        System.out.println("Using engine " + key);
    }

    return _engines.get(key);
}

我想确保每个引擎都有其单独的计数器和文件列表。但是，在我运行以下代码后，它们似乎共享计数器和列表：

TestFileEngine.getInstance("http").generateFile();
TestFileEngine.getInstance("http").generateFile();
TestFileEngine.getInstance("http").generateFile();
TestFileEngine.getInstance("ftp").generateFile();
TestFileEngine.getInstance("ftp").generateFile();

System.out.println(TestFileEngine.getInstance("http").findFileByName("4.txt").getFileName());
System.out.println(TestFileEngine.getInstance("ftp").findFileByName("2.txt"));

安慰：

Create engine http
Generated file 1.txt
Using engine http
Generated file 2.txt
Using engine http
Generated file 3.txt
Create engine ftp
Generated file 4.txt
Using engine ftp
Generated file 5.txt
Using engine http
4.txt
Using engine ftp
null

我应该如何处理counter和generatedFileList字段，以便TestFileEngine从 Multiton 创建的每个字段完全分开？

4

1 回答 1

2

static实际上，您使用修饰符声明了该类的 3 个字段：

private static int counter = 0;
private static List<GeneratedFile> generatedFileList;
private static final Map<String, TestFileEngine> _engines = new HashMap<String, TestFileEngine>();

static字段在当前类的所有实例之间共享。
你想要那个engines领域。
但是您不希望必须附加到特定实例的forcounter和字段。所以让它们成为实例字段而不是.generatedFileListTestFileEnginestatic

作为旁注，int默认情况下，字段的值是0，您应该避免_为不属于命名约定的变量添加前缀。
所以你可以写：

private int counter;
private List<GeneratedFile> generatedFileList;
private static final Map<String, TestFileEngine> engines = new HashMap<String, TestFileEngine>();
于 2017-10-25T14:50:26.357 回答