What kind of Java type is "[B"?

Question

I am trying to get MD5 encrypted pass from MySQL DB via Java code (Hibernate). But I cant get neither String nor any reasonable Java type.

The only thing I am getting is this unhelpful message: java.lang.ClassCastException: [B cannot be cast to com.mysql.jdbc.Blob (or whatever Java type I try cast to).

Here is my method:

public void testCrypto() {
        session.beginTransaction();
        // creates native SQL query
        // uses native MySQL's MD5 crypto
        final Blob pass = (Blob) session.createSQLQuery("SELECT MD5('somePass')")
            .list().get(0);
        session.getTransaction().commit();
}

Here is full stack trace:

java.lang.ClassCastException: [B cannot be cast to com.mysql.jdbc.Blob
    at domain.DatabaseTest.testCrypto(DatabaseTest.java:57)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at junit.framework.TestCase.runTest(TestCase.java:168)
    at junit.framework.TestCase.runBare(TestCase.java:134)
    at junit.framework.TestResult$1.protect(TestResult.java:110)
    at junit.framework.TestResult.runProtected(TestResult.java:128)
    at junit.framework.TestResult.run(TestResult.java:113)
    at junit.framework.TestCase.run(TestCase.java:124)
    at junit.framework.TestSuite.runTest(TestSuite.java:232)
    at junit.framework.TestSuite.run(TestSuite.java:227)
    at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:83)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

score 59 · Accepted Answer · edited Nov 26 '18 at 22:29

59

That my friend is an array of bytes. In JNI, [B is used to describe an array ([) of bytes (B). An array of ints is [I etc.

You can get a bit more information on field descriptors here:
JNI Types and Data Structures (Table 3-2 should be what you are looking for).

edited Nov 26 '18 at 22:29

Bruno Ferreira

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answered Jan 05 '11 at 17:05

rfeak

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Thank you, you have been most helpful! – Xorty Jan 05 '11 at 17:19
1

@rfeak the link outdated. Can you post a new link? – Kasnady Jul 24 '18 at 01:48
@Kasnady I have fixed the links :) – Bruno Ferreira Nov 19 '18 at 13:37

Sean Patrick Floyd · Answer 2 · 2011-01-05T17:14:48.367

9

It's the class name of byte[].class. Try this:

System.out.println(byte[].class.getName());

Output (you guessed it):

[B

And if you want to access the readable name, use Class.getCanonicalName():

System.out.println(byte[].class.getCanonicalName());

Output:

byte[]

edited Jan 05 '11 at 17:14

answered Jan 05 '11 at 17:09

Sean Patrick Floyd

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score 4 · Answer 3 · answered Jan 05 '11 at 17:15

4

As the other answers state, that is a byte array.

If you want to get a string from a byte array, use the String constructor:

public void testCrypto()
{
        session.beginTransaction();
        // creates native SQL query
        // uses native MySQL's MD5 crypto
        final String pass = new String(session.createSQLQuery("SELECT MD5('somePass')")
            .list().get(0));
        session.getTransaction().commit();
}

answered Jan 05 '11 at 17:15

Evan Mulawski

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great, but this is really strange behavior! This String is exact same as what prints mysql console. But pass.getBytes().length says its 32, and MD5 should be 128bits. Where did I lost clue? – Xorty Jan 05 '11 at 17:21
1

@Xorty: There is a difference between measuring encryption strength and the length of a String. MD5 will convert any input to a string that has no more and no less than 32 hex characters. – Evan Mulawski Jan 05 '11 at 17:25

score 3 · Answer 4 · answered Jan 05 '11 at 17:06

3

[B is the encoded type name for a byte array (byte[]), which should normally only appear in type signature strings, as its not a valid type name.

answered Jan 05 '11 at 17:06

Chris Dodd

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What kind of Java type is "[B"?

4 Answers4

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