This program shows how to read ROOT files (https://root.cern/) using Java. It illustrates problems with the compatibility of JDK versions. This example shows that compiling this code using JDK 1.5 creates a functional package that can read the file "Example.root". The JDK 1.5 compiled package can be executed on new versions of JDK 1.8 - JDK 15. However, the package compiled using JDK 1.6 (and above) creates a jar file that cannot be used for reading such files.
This compatibility issue affects several projects: http://java.freehep.org/freehep-rootio/ (original library), Jas3 http://jas.freehep.org/jas3/, Root4j https://github.com/diana-hep/root4j, Jas4pp https://atlaswww.hep.anl.gov/asc/jas4pp/, DataMelt https://datamelt.org etc. The problem is likely related to the Apache-BCEL library and may affect other projects that rely on dynamic creation of Java classes.
The issue is visible on bugs.java.com under JDK-8261887 ticket number.
git clone [email protected]:chekanov/RootIOreader.git
cd RootIOreader
This assumes Linux and installed JDK 1.8 that supports the JDK target/source version 1.5. Compile the package using JDK target=1.5:
ant
(the ant build tool should be installed). This creates the file "rootio.jar". Then run this package:
ant run
It will read objects from the file "Example.root".
Now change the target JDK version to 1.8. To do this, modify the line:
<property name="jdkversion" value="1.5" />
to
<property name="jdkversion" value="1.8" />
in the file "build.xml" (it can be also 1.6, 1.7). Then compile the package again and run it:
ant clean; ant; ant run
It will return the error:
Object hep.io.root.proxy.TString has length=1025
[java] Exception in thread "main" java.lang.ClassFormatError: Invalid code attribute name index 29 in class file hep/io/root/proxy/TString
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
[java] at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1016)
[java] at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:877)
[java] at hep.io.root.core.RootClassLoader.findClass(RootClassLoader.java:97)
[java] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
[java] at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
[java] at hep.io.root.core.GenericRootClass.getClass(GenericRootClass.java:143)
[java] at hep.io.root.core.GenericRootClass.getProxyClass(GenericRootClass.java:162)
[java] at hep.io.root.core.GenericRootClass.newInstance(GenericRootClass.java:97)
[java] at hep.io.root.core.RootInputStream.readObject(RootInputStream.java:354)
[java] at hep.io.root.core.FastInputStream.readObject(FastInputStream.java:269)
[java] at hep.io.root.RootFileReader.init(RootFileReader.java:226)
[java] at hep.io.root.RootFileReader.init(RootFileReader.java:157)
[java] at hep.io.root.RootFileReader.<init>(RootFileReader.java:137)
[java] at rootio.FileRoot.<init>(FileRoot.java:61)
[java] at rootio.Main.main(Main.java:27)
Note the object length 1025 is different from 1003 when using JDK 1.5 compiled version.
If you use JDK13 - JDK15 versions for the above tests, the error message when compiling and running the code is different:
Object hep.io.root.proxy.TString has length=1025
[java] Exception in thread "main" java.lang.ClassFormatError: LineNumberTable attribute has wrong length in class file hep/io/root/proxy/TString
[java] at java.base/java.lang.ClassLoader.defineClass1(Native Method)
...
Conclusion: Only JDK 1.5 can create a functional program that can read the file Example.root. Since JDK 1.8 (that supports the JDK 1.5 target) will be discontinued, this package cannot be used for future Java versions.
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