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2016년 3월 24일 목요일

Java 7 try-with-resources error

up vote17down votefavorite
8
I'm trying to understand how the new try-with-resources statement works by recreating it using regular try-catch-finally statements. Given the following test class using Java 7 try-with-resources:
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class TryWithResources {
    public static void main(String[] args) {
        try (GZIPOutputStream gzip = new GZIPOutputStream(System.out)) {
            gzip.write("TEST".getBytes("UTF-8"));
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}
How would you rewrite this class to use try-catch-finally statements which produces exactly the same bytecode as the try-with-resources statement produces? Also, same question when two resources are used, as in the following example:
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class TryWithResources2 {
    public static void main(String[] args) {
        try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
                GZIPOutputStream gzip = new GZIPOutputStream(baos)) {
            gzip.write("TEST".getBytes("UTF-8"));
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}
  
shareimprove this question
  
compile the class and then decompile with non 1.7 aware decompiler (any decompiler, pretty much). This will give you rough equivalent of pre-1.7 code. – Denis Tulskiy Oct 22 '11 at 14:15
9
To share valuable information with the community (see Jeff Atwood's blog, It’s OK to Ask and Answer Your Own Questions). – Dan Cruz Oct 22 '11 at 14:19 
  
What's exactly valuable, besides (slightly misguided) INVOKE_DYNAMIC java7 offers nothing new. – bestsssOct 22 '11 at 17:16
  
@bestsss - even if you don't like Java 7, the information is valuable to other people who do. Or who need that information irrespective of their views on the language. – Stephen C Oct 24 '11 at 1:24 
  
@Stephen, the information is readily available in the first draft about try-with-resource (jsr 334), here for instance: download.oracle.com/otndocs/jcp/enhancements-0.75-edr-oth-JSpec (this aint the very 1st, though). I just do not see anything valuable. As for java7: it indeed offers nothing new, even invoke_dynamic is available w/ java6... besides the hotspot version is newer than 6_29 and has some bugfixes for G1 GC. – bestsss Oct 24 '11 at 18:37

2 Answers


up vote22down voteaccepted
For the TryWithResources class, the following class produces equivalent bytecode as the try-with-resources:
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class TryCatchFinally {
    public static void main(String[] args) {
        try {
            final GZIPOutputStream gzip = new GZIPOutputStream(System.out);
            Throwable gzipEx = null;
            try {
                gzip.write("TEST".getBytes("UTF-8"));
            } catch (Throwable t) {
                gzipEx = t;
                throw t;
            } finally {
                if (gzip != null) {
                    if (gzipEx != null) {
                        try {
                            gzip.close();
                        } catch (Throwable t) {
                            gzipEx.addSuppressed(t);
                        }
                    } else {
                        gzip.close();
                    }
                }
            }
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}
Using Sun JDK 1.7.0, the bytecode and exception tables for the main method in both TryWithResources and TryCatchFinally classes is:
  stack=3, locals=6, args_size=1
     0: new           #2                  // class java/util/zip/GZIPOutputStream
     3: dup           
     4: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
     7: invokespecial #4                  // Method java/util/zip/GZIPOutputStream."<init>":(Ljava/io/OutputStream;)V
    10: astore_1      
    11: aconst_null   
    12: astore_2      
    13: aload_1       
    14: ldc           #5                  // String TEST
    16: ldc           #6                  // String UTF-8
    18: invokevirtual #7                  // Method java/lang/String.getBytes:(Ljava/lang/String;)[B
    21: invokevirtual #8                  // Method java/util/zip/GZIPOutputStream.write:([B)V
    24: aload_1       
    25: ifnull        95
    28: aload_2       
    29: ifnull        48
    32: aload_1       
    33: invokevirtual #9                  // Method java/util/zip/GZIPOutputStream.close:()V
    36: goto          95
    39: astore_3      
    40: aload_2       
    41: aload_3       
    42: invokevirtual #11                 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
    45: goto          95
    48: aload_1       
    49: invokevirtual #9                  // Method java/util/zip/GZIPOutputStream.close:()V
    52: goto          95
    55: astore_3      
    56: aload_3       
    57: astore_2      
    58: aload_3       
    59: athrow        
    60: astore        4
    62: aload_1       
    63: ifnull        92
    66: aload_2       
    67: ifnull        88
    70: aload_1       
    71: invokevirtual #9                  // Method java/util/zip/GZIPOutputStream.close:()V
    74: goto          92
    77: astore        5
    79: aload_2       
    80: aload         5
    82: invokevirtual #11                 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
    85: goto          92
    88: aload_1       
    89: invokevirtual #9                  // Method java/util/zip/GZIPOutputStream.close:()V
    92: aload         4
    94: athrow        
    95: goto          103
    98: astore_1      
    99: aload_1       
   100: invokevirtual #13                 // Method java/io/IOException.printStackTrace:()V
   103: return        
  Exception table:
     from    to  target type
        32    36    39   Class java/lang/Throwable
        13    24    55   Class java/lang/Throwable
        13    24    60   any
        70    74    77   Class java/lang/Throwable
        55    62    60   any
         0    95    98   Class java/io/IOException
For the TryWithResources2 class, the following class produces equivalent bytecode as the try-with-resources:
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class TryCatchFinally2 {
    public static void main(String[] args) {
        try {
            final ByteArrayOutputStream baos = new ByteArrayOutputStream();
            Throwable baosEx = null;
            try {
                final GZIPOutputStream gzip = new GZIPOutputStream(baos);
                Throwable gzipEx = null;
                try {
                    gzip.write("TEST".getBytes("UTF-8"));
                } catch (Throwable t) {
                    gzipEx = t;
                    throw t;
                } finally {
                    if (gzip != null) {
                        if (gzipEx != null) {
                            try {
                                gzip.close();
                            } catch (Throwable t) {
                                gzipEx.addSuppressed(t);
                            }
                        } else {
                            gzip.close();
                        }
                    }
                }
            } catch (Throwable t) {
                baosEx = t;
                throw t;
            } finally {
                if (baos != null) {
                    if (baosEx != null) {
                        try {
                            baos.close();
                        } catch (Throwable t) {
                            baosEx.addSuppressed(t);
                        }
                    } else {
                        baos.close();
                    }
                }
            }
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}
The bytecode and exception tables for the main method in both TryWithResources2 and TryCatchFinally2 classes is:
  stack=3, locals=10, args_size=1
     0: new           #2                  // class java/io/ByteArrayOutputStream
     3: dup           
     4: invokespecial #3                  // Method java/io/ByteArrayOutputStream."<init>":()V
     7: astore_1      
     8: aconst_null   
     9: astore_2      
    10: new           #4                  // class java/util/zip/GZIPOutputStream
    13: dup           
    14: aload_1       
    15: invokespecial #5                  // Method java/util/zip/GZIPOutputStream."<init>":(Ljava/io/OutputStream;)V
    18: astore_3      
    19: aconst_null   
    20: astore        4
    22: aload_3       
    23: ldc           #6                  // String TEST
    25: ldc           #7                  // String UTF-8
    27: invokevirtual #8                  // Method java/lang/String.getBytes:(Ljava/lang/String;)[B
    30: invokevirtual #9                  // Method java/util/zip/GZIPOutputStream.write:([B)V
    33: aload_3       
    34: ifnull        114
    37: aload         4
    39: ifnull        61
    42: aload_3       
    43: invokevirtual #10                 // Method java/util/zip/GZIPOutputStream.close:()V
    46: goto          114
    49: astore        5
    51: aload         4
    53: aload         5
    55: invokevirtual #12                 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
    58: goto          114
    61: aload_3       
    62: invokevirtual #10                 // Method java/util/zip/GZIPOutputStream.close:()V
    65: goto          114
    68: astore        5
    70: aload         5
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