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[Java] try-with-resources

태애니 2023. 2. 25. 15:26
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https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

 

The try-with-resources Statement (The Java™ Tutorials > Essential Java Classes > Exceptions)

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated

docs.oracle.com

 

 

try-catch 말고 

try-with-resources 자원해제 처리 문이라는게 있다고 하길래 알아보기로 했다

 

외부자원에 접근 후 자원해제시켜야할 때 이를 개선시켜준다고 한다

 

 

 

간단히 말하면 try에 자원객체를 전달하며 자동으로 코드 블록이 끝날 시 자원을 종료해주고,

finally나 catch 를 따로 작성할 필요가 없는 statement이다

 

 

대신 이때의 resource 는 AutoCloseable interface 구현체를 try(이 안에 넣어야한) 다

 

/*
 * Copyright (c) 2009, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang;

/**
 * An object that may hold resources (such as file or socket handles)
 * until it is closed. The {@link #close()} method of an {@code AutoCloseable}
 * object is called automatically when exiting a {@code
 * try}-with-resources block for which the object has been declared in
 * the resource specification header. This construction ensures prompt
 * release, avoiding resource exhaustion exceptions and errors that
 * may otherwise occur.
 *
 * @apiNote
 * <p>It is possible, and in fact common, for a base class to
 * implement AutoCloseable even though not all of its subclasses or
 * instances will hold releasable resources.  For code that must operate
 * in complete generality, or when it is known that the {@code AutoCloseable}
 * instance requires resource release, it is recommended to use {@code
 * try}-with-resources constructions. However, when using facilities such as
 * {@link java.util.stream.Stream} that support both I/O-based and
 * non-I/O-based forms, {@code try}-with-resources blocks are in
 * general unnecessary when using non-I/O-based forms.
 *
 * @author Josh Bloch
 * @since 1.7
 */
public interface AutoCloseable {
    /**
     * Closes this resource, relinquishing any underlying resources.
     * This method is invoked automatically on objects managed by the
     * {@code try}-with-resources statement.
     *
     * <p>While this interface method is declared to throw {@code
     * Exception}, implementers are <em>strongly</em> encouraged to
     * declare concrete implementations of the {@code close} method to
     * throw more specific exceptions, or to throw no exception at all
     * if the close operation cannot fail.
     *
     * <p> Cases where the close operation may fail require careful
     * attention by implementers. It is strongly advised to relinquish
     * the underlying resources and to internally <em>mark</em> the
     * resource as closed, prior to throwing the exception. The {@code
     * close} method is unlikely to be invoked more than once and so
     * this ensures that the resources are released in a timely manner.
     * Furthermore it reduces problems that could arise when the resource
     * wraps, or is wrapped, by another resource.
     *
     * <p><em>Implementers of this interface are also strongly advised
     * to not have the {@code close} method throw {@link
     * InterruptedException}.</em>
     *
     * This exception interacts with a thread's interrupted status,
     * and runtime misbehavior is likely to occur if an {@code
     * InterruptedException} is {@linkplain Throwable#addSuppressed
     * suppressed}.
     *
     * More generally, if it would cause problems for an
     * exception to be suppressed, the {@code AutoCloseable.close}
     * method should not throw it.
     *
     * <p>Note that unlike the {@link java.io.Closeable#close close}
     * method of {@link java.io.Closeable}, this {@code close} method
     * is <em>not</em> required to be idempotent.  In other words,
     * calling this {@code close} method more than once may have some
     * visible side effect, unlike {@code Closeable.close} which is
     * required to have no effect if called more than once.
     *
     * However, implementers of this interface are strongly encouraged
     * to make their {@code close} methods idempotent.
     *
     * @throws Exception if this resource cannot be closed
     */
    void close() throws Exception;
}

 

 

이렇게 구현해주고

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


class TestCodeCheck implements AutoCloseable{
    public TestCodeCheck(TestCode testCode){

    }

    @Override
    public void close() throws Exception {
        System.out.println("TestCodeCheck call");
    }
}

 

 try(TestCode tescode = new TestCode()){

    }catch(Exception e){

    }

 

 

public static void main(String[] args) {
    try(TestCode testCode = new TestCode();
        TestCodeCheck testCodeCheck = new TestCodeCheck(testCode)){

    }catch(Exception e){

    }
}

둘다 해제시키기

 

 

TestCodeCheck 만으로 다 해제시키려면

TestCodeCheck안에 TestCode 를 해제시켜주는 코드를 작성

 

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


class TestCodeCheck implements AutoCloseable{
    public TestCodeCheck(TestCode testCode){

    }

    @Override
    public void close() throws Exception {
    	this.TestCode.close();
        System.out.println("TestCodeCheck call");
    }
}

 

 

 

The following example retrieves the names of the files packaged in the zip file zipFileName and creates a text file that contains the names of these files:

public static void writeToFileZipFileContents(String zipFileName,
                                           String outputFileName)
                                           throws java.io.IOException {

    java.nio.charset.Charset charset =
         java.nio.charset.StandardCharsets.US_ASCII;
    java.nio.file.Path outputFilePath =
         java.nio.file.Paths.get(outputFileName);

    // Open zip file and create output file with 
    // try-with-resources statement

    try (
        java.util.zip.ZipFile zf =
             new java.util.zip.ZipFile(zipFileName);
        java.io.BufferedWriter writer = 
            java.nio.file.Files.newBufferedWriter(outputFilePath, charset)
    ) {
        // Enumerate each entry
        for (java.util.Enumeration entries =
                                zf.entries(); entries.hasMoreElements();) {
            // Get the entry name and write it to the output file
            String newLine = System.getProperty("line.separator");
            String zipEntryName =
                 ((java.util.zip.ZipEntry)entries.nextElement()).getName() +
                 newLine;
            writer.write(zipEntryName, 0, zipEntryName.length());
        }
    }
}

 

The following example uses a try-with-resources statement to automatically close a java.sql.Statement object:

public static void viewTable(Connection con) throws SQLException {

    String query = "select COF_NAME, SUP_ID, PRICE, SALES, TOTAL from COFFEES";

    try (Statement stmt = con.createStatement()) {
        ResultSet rs = stmt.executeQuery(query);

        while (rs.next()) {
            String coffeeName = rs.getString("COF_NAME");
            int supplierID = rs.getInt("SUP_ID");
            float price = rs.getFloat("PRICE");
            int sales = rs.getInt("SALES");
            int total = rs.getInt("TOTAL");

            System.out.println(coffeeName + ", " + supplierID + ", " + 
                               price + ", " + sales + ", " + total);
        }
    } catch (SQLException e) {
        JDBCTutorialUtilities.printSQLException(e);
    }
}
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