java – try-with-resources中的死代码警告,但不是在翻译的try-c

下面的代码使用 Java 8中引入的 try-with-resources结构.偶然的Throw()方法被声明为抛出OccasionalException,Resource的close()方法抛出一个CloseException. Eclipse(版本：Neon Release(4.6.0),Build id：20160613-1800)在标有//死代码的行上添加一个警告,表示该分支是死代码.隐含地,Eclipse确认标有//活动代码的行不是死代码.

Object tryWithResources() throws OccasionalException {
    Object value = null;
    try (Resource resource = new Resource()) {
        occasionallyThrow();
        value = new Object();
    }
    catch (CloseException e) {
        if (value == null) {
            // alive code
        }
        else {
            // dead code
        }
    }
    return value;
}

我很困惑.如果偶然的Throw()抛出其OccasionalException,那么try-with-resources应该将其作为主要异常捕获,然后尝试关闭资源.如果关闭资源会抛出一个CloseException,那么它将在OccasionalException下被抑制,因此不会捕获CloseException.因此,唯一需要捕获的CloseException是在try中的块成功完成时,这意味着该值为非null.所以看起来“死代码”实际上是活着的,而“活着代码”实际上已经死了.我不确定编译器实际上应该在这里识别出什么,但至少,这里的“死代码”似乎不应该被称为死.

更复杂的是,不使用try-with-resources表单的翻译表单根本没有标记任何死代码警告. (我非常有信心,基于14.20.3.2. Extended try-with-resources,我的翻译是正确的,但如果这里有错误,我不会感到惊讶……)

Object expandedTry() throws OccasionalException {
    Object value = null;
    try {
        Resource resource = new Resource();
        Throwable $primary = null;
        try {
            occasionallyThrow();
            value = new Object();
        }
        catch (Throwable t) {
            $primary = t;
            throw t;
        }
        finally {
            if (resource != null) {
                if ($primary != null) {
                    try {
                        resource.close();
                    }
                    catch (Throwable $suppressed) {
                        $primary.addSuppressed($suppressed);
                    }
                }
                else {
                    resource.close();
                }
            }
        }
    }
    catch (CloseException e) {
        if (value == null) {
            // alive (not dead!)
        }
        else {
            // alive
        }
    }
    return value;
}

我是否遗漏了一些东西会使if-else中的任何一个分支死于其中一个,而不是另一个？

完整代码

这是包含辅助异常类型,Resource类和顶级类定义的完整代码.

public class TestTryWithResources {

    /** Exception thrown by Resource's close() method */
    @SuppressWarnings("serial")
    static class CloseException extends Exception {}

    /** AutoCloseable declared to throw a CloseException */ 
    static class Resource implements AutoCloseable {
        @Override
        public void close() throws CloseException {}
    }

    /** An occasionally thrown exception */
    @SuppressWarnings("serial")
    static class OccasionalException extends Exception {}

    /** Method declared to throw an occasional exception */
    void occasionallyThrow() throws OccasionalException {}

    /*
     * Method using try-with-resources.  Eclipse warns that the 
     * portion marked with "// dead code" is Dead code.
     */
    Object tryWithResources() throws OccasionalException {
        Object value = null;
        try (Resource resource = new Resource()) {
            occasionallyThrow();
            value = new Object();
        }
        catch (CloseException e) {
            if (value == null) {
                // alive code
            }
            else {
                // dead code
            }
        }
        return value;
    }

    /*
     * Method not using try-with-resources.  This is the translation
     * of the try-with-resources in tryWithResources,according to 
     * [14.20.3 try-with-resources][1].  Eclipse does not warn about 
     * any of the code being Dead code.
     * 
     * [1]: https://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.20.3 
     */
    Object expandedTry() throws OccasionalException {
        Object value = null;
        try {
            Resource resource = new Resource();
            Throwable $primary = null;
            try {
                occasionallyThrow();
                value = new Object();
            }
            catch (Throwable t) {
                $primary = t;
                throw t;
            }
            finally {
                if (resource != null) {
                    if ($primary != null) {
                        try {
                            resource.close();
                        }
                        catch (Throwable $suppressed) {
                            $primary.addSuppressed($suppressed);
                        }
                    }
                    else {
                        resource.close();
                    }
                }
            }
        }
        catch (CloseException e) {
            if (value == null) {
                // alive
            }
            else {
                // alive
            }
        }
        return value;
    }
}

回应评论

Amin J’s answer建议在设置值之后使用资源来改变Eclipse的代码分析.但这不起作用.在使用资源之后,例如通过打印它,Luna和Neon中仍然存在死码警告：

Object tryWithResources() throws OccasionalException {
    Object value = null;
    try (Resource resource = new Resource()) {
        occasionallyThrow();
        value = new Object();
        resource.someMethod(); // using the resource,so eclipse thinks it's not closed yet (correctly)
    }
    catch (CloseException e) {
        if (value == null) {
            // alive code
        }
        else {
            // dead code
        }
    }
    return value;
}

Object val = null;

        try (BufferedReader reader = new BufferedReader(new FileReader("C:file.txt"))) {
            val = new Object();
            System.out.println("got here");
            reader.readLine();
        }
        catch(IOException e){
            System.out.println("io ex");
            if ( val == null){

            }
            else{

            }
        }