DDevelopment & Design

GWT Hosted mode on 64bit linux

GWT for linux is build against 32bit architecture. It contains some SWT/GTK 32bit modules. So if you try to run it with 64bit java it failsException in thread “main” java.lang.UnsatisfiedLinkError: /opt/tools/sdk/gwt/gwt-linux-1.5.3/libswt-pi-gtk-3235….

GWT for linux is build against 32bit architecture. It contains some SWT/GTK 32bit modules. So if you try to run it with 64bit java it fails

Exception in thread “main” java.lang.UnsatisfiedLinkError: /opt/tools/sdk/gwt/gwt-linux-1.5.3/libswt-pi-gtk-3235.so: /opt/tools/sdk/gwt/gwt-linux-1.5.3/libswt-pi-gtk-3235.so: wrong ELF class: ELFCLASS32 (Possible cause: architecture word width mismatch)
    at java.lang.ClassLoader$NativeLibrary.load(Native Method)
    at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1807)
    at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1703)
    at java.lang.Runtime.load0(Runtime.java:770)
    at java.lang.System.load(System.java:1003)
    at org.eclipse.swt.internal.Library.loadLibrary(Library.java:132)
    at org.eclipse.swt.internal.gtk.OS.(OS.java:22)
    at org.eclipse.swt.internal.Converter.wcsToMbcs(Converter.java:63)
    at org.eclipse.swt.internal.Converter.wcsToMbcs(Converter.java:54)
    at org.eclipse.swt.widgets.Display.(Display.java:126)
    at com.google.gwt.dev.GWTShell.(GWTShell.java:301)
Could not find the main class: com.google.gwt.dev.GWTShell.  Program will exit.
[INFO] ————————————————————————
[ERROR] BUILD ERROR

You have two choices. Find 64bit SWT/GTK modules with same build version (3235 in this case) – good luck! or download 32bit JRE.

I chose second option and it took me 3 mins to resolve the problem.
Find proper JRE version on Oracle site I suggest bin file instead rpm. It unpacks jre to own dir. Move that directory to some convenient location (it doesn’t matter where). Edit gwt.properties *and set java.executable* to java exec located in 32bit JRE.
Now run your GWT hosted mode and be unstoppable developer!

You may have some warnings from GTK, such as

/usr/lib/gtk-2.0/2.10.0/menuproxies/libappmenu.so: wrong ELF class: ELFCLASS64
(GWT:351): Gtk-WARNING **: Failed to load type module: /usr/lib/gtk-2.0/2.10.0
/menuproxies/libappmenu.so
**

But it has no consequences for me, so far…

Some sources say that you should set an environment var:

export LIBXCB_ALLOW_SLOPPY_LOCK=1

to block bugs in X display layer but I don’t know what does it mean :)

In this case I had to hadle with GWT version 1.5.3 (old corporate project)

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Private fields and methods are not private in groovy

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
private String name
public  String surname

private Person() {}

private String signature() { "${name?.substring(0, 1)}. $surname" }

public String toString() { "I am $name $surname" }
}

How is this class interpreted with Java?

  1. Person has private constructor that cannot be accessed
  2. Field "name" is private and cannot be accessed
  3. Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
def person = new Person()   // constructor is private - compilation error in Java
println(person.toString())

    person.@name = 'Mike'       // access name field directly - compilation error in Java
println(person.toString())

    person.name = 'John'        // there is a setter generated by groovy
println(person.toString())

    person.@surname = 'Foo'     // access surname field directly
println(person.toString())

    person.surname = 'Bar'      // access auto-generated setter
println(person.toString())

println(person.signature()) // call private method - compilation error in Java
}

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: http://jira.codehaus.org/browse/GROOVY-1875. It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!