Java `InvocationTargetException` 通过反射进行类实例化

Question

我遇到的问题是:我有一系列通过注释收集的类。它们都位于同一个文件夹中，如果它们具有特定的注释，它们将通过 Reflections library 实例化。 。当这些类被实例化时，有一个静态初始化程序调用静态工厂，它构建一些结构。当尝试获取工厂创建的对象时，Java 将抛出 InitationTargetException 错误。更具体地说，当我输出 ITE 的堆栈跟踪时，它直接指向向工厂请求对象的静态初始化程序。

下面是我用来复制问题的代码。

我有一个注释:InferenceRule.java

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface funRule {
    String ruleName();
    String ruleType();
    String analyze() default "node";
}

然后，我将该注释应用于包中的一些类inference.rules:

@InferenceRule(ruleName = "assign", ruleType = "term")
public class Assign extends NodeAnalyzer {
    public Assign() {super();}
    public Assign(String... args) { super(args); }
    public Rule gatherAllCOnstraints(InstructionNode node) {
        // use the Identifier object here.
    }
    // rest of class here
}

NodeAnalyzer 类，上面 Assign 类的父类(super class):

public abstract class NodeAnalyzer {
    protected Identifier identifier;

    protected NodeAnalyzer() {
        // Construct things here
    }

    protected NodeAnalyzer(String... args) {
        // Construct other things here
    }

    // Construct common things here
    {
        this.identifier = IdentifierFactory.getIdentifier();
    }
    // rest of class here
}

Assign 类在 Inference 类中实例化，如下所述:

public class Inference {
    public final String NODE_ANALYSIS = "NODE";
    public static final String INFERENCE_PACKAGE = "inference.rules";
    private final Map<String, NodeAnalyzer> nodeAnalyzer = new HashMap<>();
    private final Map<String, EdgeAnalyzer> edgeAnalyzer = new HashMap<>();
    public Inference() {

    }
    // other non-interesting things here

    private void loadRules() {
        Reflections reflection = new Reflections(INFERENCE_PACKAGE);
        Set<Class<?>> annotated = reflection.getTypesAnnotatedWith(InferenceRule.class);

        for(Class<?> clazz : annotated) {
            try {
                String name = clazz.getAnnotation(InferenceRule.class).ruleName();
                String type = clazz.getAnnotation(InferenceRule.class).ruleType();
                String analyze = clazz.getAnnotation(InferenceRule.class).analyze();
                if (StringUtils.equalsIgnoreCase(analyze, NODE_ANALYSIS)) {
                    final NodeAnalyzer newInstance = (NodeAnalyzer) clazz.getConstructor(InferenceType.class).newInstance(InferenceType.valueOf(type));
                    this.nodeAnalyzer.put(name, newInstance);
                }
                // handle other cases...
            } catch(InvocationTargetException ite) {
                // For debugging, only
                ite.printStackTrace();
                logger.error(ite.getCause.getMessage());
                logger.error(ite.getTargetException.getMessage());
            }
        }
    }
}

可以看到，从Assign和NodeAnalyzer中的实例化路径来看，它必须调用IdentifierFactory类:

public class IdentifierFactory {
    private static final Identifier identifier;
    static {
        if (ConfigFactory.getConfig().isDebEnabled()) {
            identifier = new DBIdentifier();
        } else {
            identifier = new NaiveIdentifier();
        }
    }

    public static Identifier getIdentifier() {
        return identifier;
    }
}

NaiveIdentifier 类:

public class NaiveIdentifier {
    private Set<Integer> unknowns = new HashSet<Integer>() {{
        unknowns.add(0);
        // add more here.
    };

    public NaiveIdentifier() {} // empty default constructor
}

ConfigFactory 类遵循与 IdentifierFactory 类类似的模式。它根据某些输入构建配置。

抛出的确切异常如下所示:

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at phases.inference.Inference.loadRules(Inference.java:197)
    at phases.inference.Inference.<init>(Inference.java:76)
    at phases.PhaseFacade$PHASES.getPhase(PhaseFacade.java:27)
    at phases.PhaseFacade.<init>(PhaseFacade.java:42)
    at compilation.Compiler.runPhases(Compiler.java:126)
    at compilation.Compiler.runAllOps(Compiler.java:118)
    at Main.main(Main.java:45)
Caused by: java.lang.ExceptionInInitializerError
    at phases.inference.rules.NodeAnalyzer.<init>(NodeAnalyzer.java:35)
    at phases.inference.rules.Assign.<init>(Assign.java:22)
    ... 11 more
Caused by: java.lang.NullPointerException
    at typesystem.identification.NaiveIdentifier$1.<init>(NaiveIdentifier.java:23)
    at typesystem.identification.NaiveIdentifier.<init>(NaiveIdentifier.java:22)
    at typesystem.identification.IdentifierFactory.<clinit>(IdentifierFactory.java:25)
    ... 13 more

和:

java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at phases.inference.Inference.loadRules(Inference.java:197)
    at phases.inference.Inference.<init>(Inference.java:76)
    at phases.PhaseFacade$PHASES.getPhase(PhaseFacade.java:27)
    at phases.PhaseFacade.<init>(PhaseFacade.java:42)
    at compilation.Compiler.runPhases(Compiler.java:126)
    at compilation.Compiler.runAllOps(Compiler.java:118)
    at Main.main(Main.java:45)
Caused by: java.lang.NoClassDefFoundError: Could not initialize class typesystem.identification.IdentifierFactory
    at phases.inference.rules.NodeAnalyzer.<init>(NodeAnalyzer.java:35)
    at phases.inference.rules.Assign.<init>(Assign.java:18)
    ... 11 more

从这些来看，我无法充分辨别根本原因是什么。更复杂的是，我尝试使用其他输入文件来运行它，并且它在这些文件上运行得很好。

Answer 1

这段代码

public class NaiveIdentifier {
    private Set<Integer> unknowns = new HashSet<Integer>() {{
        unknowns.add(0);
        // add more here.
    }}; // ( <- added missing brace here)

    public NaiveIdentifier() {} // empty default constructor
}

正在使用“双花括号初始化”反模式。通常，这种反模式用于节省源代码中的一些输入:

public class NaiveIdentifier {
    private Set<Integer> unknowns = new HashSet<Integer>() {{
        // yeah, we saved writing the nine characters "unknowns."
        add(0);
        // add more here.
    }};

    public NaiveIdentifier() {} // empty default constructor
}

以创建集合类的新子类为代价，并且可能会由于内部类保存对其外部类实例的引用而导致内存泄漏，如 this Q&A 中所述。 .

具有讽刺意味的是，您没有省略字符 unknowns. ，因此不仅没有利用这种反模式，而且还创建了这个错误，因为您正在访问应该使用集合的构造函数中构造的集合实例初始化的字段。换句话说，您的代码相当于以下代码:

public class NaiveIdentifier {
    private Set<Integer> unknowns;
    {
      Set<Integer> temp = new HashSet<Integer>() {{
        unknowns.add(0);
        // add more here.
      }};
      unknowns = temp;
    }

    public NaiveIdentifier() {} // empty default constructor
}

这清楚地表明了为什么此代码会失败并显示 NullPointerException .

您可以通过一致使用反模式来解决此问题，即删除 unknowns.字符来更改对父类(super class)调用的外部实例字段访问(如上面的第二个代码示例中)，但是，现在字符已经存在，您可以轻松更改代码以使用干净的初始化程序而无需反模式:

public class NaiveIdentifier {
    private Set<Integer> unknowns = new HashSet<Integer>();
    {
        unknowns.add(0);
        // add more here.
    }

    public NaiveIdentifier() {} // empty default constructor
}

使用单花括号时，您不会创建 HashSet 的内部类子类，但只是定义一个将添加到 NaiveIdentifier 的构造函数中的初始值设定项，按照预期的程序文本顺序执行，首先是初始化器unknowns = new HashSet<Integer>() ，然后 unknowns.add(…);声明。

对于简单的初始化语句，您可以考虑替代方案

public class NaiveIdentifier {
    private Set<Integer> unknowns = new HashSet<>(Arrays.asList(0, 1, 2, 3 …));

    public NaiveIdentifier() {} // empty default constructor
}