Java: NoSuchMethodException with reflection when method clearly exists

Question

I've been trying to use Reflection in Java, Here's my code:

String charsetName = "UTF-16LE";
java.nio.charset.CharsetDecoder cd = Charset.forName(charsetName).newDecoder();

Class c = cd.getClass();

Class[] paramTypes = new Class[] {ByteBuffer.class, CharBuffer.class };

try {
     Method method = c.getDeclaredMethod("decodeLoop", paramTypes);
     method.setAccessible(true);
     assertTrue(true);
} catch (NoSuchMethodException | SecurityException e) {
     e.printStackTrace();
     assertTrue(false);
}

Method clearly exists. Java source:

package java.nio.charset;

public abstract class CharsetDecoder {
...
   protected abstract CoderResult decodeLoop(ByteBuffer in,
                                          CharBuffer out);
...
}

The output:

java.lang.NoSuchMethodException:   sun.nio.cs.UTF_16LE$Decoder.decodeLoop(java.nio.ByteBuffer, java.nio.CharBuffer)
at java.lang.Class.getDeclaredMethod(Class.java:2130)
at com.krasutski.AppTest.testDeclaredMethod(AppTest.java:227)
...

if I use a parameter charsetName as

"UTF-16LE" - Exception NoSuchMethodException
"UTF-16BE" - Exception NoSuchMethodException
"UTF-8" - Very good
"cp1252" - Very good

How I'm supposed to fix this?

Why are you doing this at all? The result of the reflective invocation of decodeLoop(in, out) is exactly the same as using the official API call cd.onMalformedInput(CodingErrorAction.REPORT).decode(in, out, false)… — Holger
– Holger, Commented Sep 29, 2016 at 15:42

Jon Skeet · Accepted Answer · 2016-09-26 12:35:49Z

4

You're calling getDeclaredMethod on the actual type of cd, whereas it's declared in CharsetDecoder. That can't be the actual type of cd given that it's an abstract class.

Just change this:

Class c = cd.getClass();

to

Class c = CharsetDecoder.class;

The exception goes away at that point. If it's working for UTF-8 and cp1252, that suggests that the classes that are used for that also declare decodeLoop, whereas for UTF-16LE and UTF-16BE they're potentially inherited.

answered Sep 26, 2016 at 12:35

Jon Skeet

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