java继承和TryWithResources

Question

假设有两个类实现AutoCloseable接口，如下所示：

public class Closing1 implements AutoCloseable {

private boolean closed;

@Override
public void close() throws Exception {
    if (closed) {
        throw new Exception("Closed Already");
    }
    this.closed = true;
    System.out.println("Closing1 closed");
}

public boolean isClosed() {
    return closed;
}

}

及

public class Closing2 implements AutoCloseable {

private Closing1 cl1;

public Closing2(Closing1 cl1) {
    this.cl1 = cl1;
}

@Override
public void close() throws Exception {
    if(!cl1.isClosed()) {
        throw new Exception("Closing1 not closed");
    }
    System.out.println("Closing2 closed");
}

}

我发现try-with-resources的所有变体都会导致异常！这里有没有我遗漏的东西，或者这只是TWR的设计方式

        try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
            System.out.println("Done");
        } //Exception while auto closing C2

或

        try(Closing1 c1 = new Closing1();Closing2 c2 = new Closing2(c1)){
            System.out.println("Done");
            c1.close();
        } // exception while auto closing c1

Answer 1

首先尝试使用资源，https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html
正如第一个示例所示：

try (BufferedReader br = new BufferedReader(new FileReader(path))) {
    return br.readLine();
}

人们不一定会说出链中的每件事

除非您明确地需要c1来完成某些事情（而不是结束），否则在现实生活中，您的代码片段更像

try(Closing2 c2 = new Closing2(new Closing1())){
    System.out.println("Done");
}

而且您肯定不会在try块中调用c1.close()，因为根本没有c1

记住这一点，因为包含的c1未关闭而从c2引发异常是完全错误的，实际上c2拥有Closing1对象，并且应该对其调用close()：

class Close1 implements AutoCloseable {
    @Override
    public void close() throws Exception {
        System.out.println("Closing c1");
    }
}

class Close2 implements AutoCloseable {
    Close1 c1;
    Close2(Close1 c1) {
        this.c1=c1;
    }

    @Override
    public void close() throws Exception {
        System.out.print("Closing c1 from c2: ");
        c1.close();
        System.out.println("Closing c2");
    }
}

void test() {
    System.out.println("Before try block");
    try(Close2 c2=new Close2(new Close1())) {
        System.out.println("In try block");
    }
    catch(Exception ex) {
        System.out.println("Exception: "+ex);
    }
    finally {
        System.out.println("In finally block");
    }
    System.out.println("After try block");
}

但是，如果有人给c1命名，它将被关闭两次，这就是幂等性出现的地方，正如已经有人建议的：

System.out.println("Before try block");
try(Close1 c1 = new Close1(); Close2 c2 = new Close2(c1)){
    System.out.println("In try block");
}
catch(Exception ex){
    System.out.println("Exception: "+ex);
}
finally{
    System.out.println("In finally block");
}
System.out.println("After try block");

正如前面提到的BufferedReader，这是它的close()方法：

public void close() throws IOException {
    synchronized (lock) {
        if (in == null)
            return;
        try {
            in.close();
        } finally {
            in = null;
            cb = null;
        }
    }
}

如果它有in，它将被关闭并为null（在finally块中，即使发生异常也会发生），所有这些都在线程安全块中。（cb只是一个字符数组，它也会变为null，从而稍微简化了垃圾收集器的使用寿命）。由于将finally块中的所有内容置零，因此对同一方法的任何额外调用都不会做任何事情（除了在锁上同步一段时间之外）

Answer 2

Try with resources将按与其声明相反的顺序关闭资源。这意味着将首先调用c2.close()，这将在编码异常时引发异常