Why close method of java.lang.AutoCloseable throws Exception, but close method of java.io.Closeable throws IOException?

问题

I was reading this link for try-with-resources and it says:

The close method of the Closeable interface throws exceptions of type IOException while the close method of the AutoCloseable interface throws exceptions of type Exception.

But why? The close method of AutoCloseable could have also thrown IOException is there any example that support that close method of AutoCloseable must throw exceptions of type Exception

回答1:

The AutoClosable interface is located in java.lang and is intended to be applied to any resource that needs to be closed 'automatically' (try-with-resources). The AutoClosable must not be an io releated resource. So the interface can not make any assumption of a concrete exception.

On the other hand Closable is located in java.io and extends AutoClosable, because a Closable is an AutoClosable for io resources. Therefore it declares that IOExceptions can be thrown on close.

For example... a java.sql.Connection is an AutoClosable because it's close method throws SQLException and a SQLException is not an IOException. Think about in memory DBs and it makes sense that closing an sql connection must not throw an IOException.

EDIT

answered one more doubt i.e. why AutoClosable is kept under java.lang package. Thanks.

I think it is located in java.lang because try-with-resources was introduced as a language feature in Java 1.7. Thus java.lang

回答2:

Closeable extends AutoCloseable, but there could be other particular interfaces that extend this interface. E.g.:

public interface MyCloseable extends AutoCloseable { 
    void close() throws RuntimeException; 
}

They wanted to have an interface that can be used in many cases, and this is why they decided to use Exception because it also works for other types of exceptions.

回答3:

Further to being able to throw some other types of exceptions than IOException one beautiful and common use case can be easily overseen:

One can override the interface to have no throws declaration at all, thus permitting try being written without explicit exception handling.

In our code we have an interface Searcher declared in the following way

public interface Searcher<V> extends AutoCloseable {

    Stream<V> search();

    @Override
    void close();
}

This permits the following use of Searcher instances:

try (Searcher<Datatype> dataTypeSearcher = new DataTypeSearcher(query)) {
    return dataTypeSearcher.search();
}
// without any catch statements

If no throws declaration was present on AutoCloseable, the above would be the only usage as it would not be possible to override the AutoCloseable interface throwing an exception not declared on the parent. The way it is currently done, both options are possible.

来源：https://stackoverflow.com/questions/25959779/why-close-method-of-java-lang-autocloseable-throws-exception-but-close-method-o

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java

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try-with-resources