Develop Reference

Can Java delete all files and folders of a computer?

I'm just curious (but not willing to try on my computer) what would happen if I run this code on Java?
private static void deleteAll(File file)
{
for(File f : file.listFiles())
{
if(f.isFile())
{
f.delete();
}
else
{
deleteAll(f);
f.delete();
}
}
}
public static void main(String[] args) {
File file = new File("/home");
deleteAll(file);
}
Is this equivalent to rm -rf / in linux and will delete everything on my computer? Or will the operating system stop me from deleting System files?
Thank you so much!

Okay this got my interest and I tried it on a lxc container. Now it wasn't a scientific test, but better than nothing right.
Well, I would say that I got mixed results. If you try to delete files with java normally, not system ones, then you are gonna do your work. Once you start killing your system, then thnings change. Java won't allow you to delete anything from your root directory, but I did successfuly delete my user folder (/home/ubuntusuer). I didn't corrupt the system, alas.
There is a mechanism to stop you from deleting a whole system. So the behaviour will not be exactly the same as compared to rm -rf /, but you can still remove a lot of stuff with it.

How to replace java code block to another code block automatically or pragramatically?

I have many java source files in which I want to change a code block to another code block in every java files that I have in my project.
e.g.
My project has about 300 java source file and every file has this code block
while (p.getCurrentToken() != JsonToken.END_OBJECT) {
p.nextToken();
field = p.getCurrentName();
obj.populateFromJsonString(field, p);
}
I want to replace this code block to
while (p.nextToken() != null) {
field = p.getCurrentName();
if(field!=null){
obj.populateFromJsonString(field, p);
}
}
in all the java files. object obj in code block is different for different files
I know somehow this can be done by text processing with sed and awk. I am thinking for alternate solution the way compilers read and parse java source file. there may be some application based on this way for solving my problem.
Please share way or application to solve this problem. sed/awk based solutions are also welcome.

It's probably best to use an IDE to do this, but if you really wanted to use sed, put this in a file called j.sed
/while (p\.getCurrentToken() != JsonToken\.END_OBJECT) {/{
:loop
n
/}/b
s/field = p\.getCurrentName();/if (field!=null) {/
s/p\.nextToken();/field = p\.getCurrentName();/
s/...\.populateFromJsonString(field, p);/\t&\n\t}/
b loop
}
Then use in the command line
find . -type f -name '*.java' -exec sed -i.bak -f j.sed {} \;
This will on input:
while (p.getCurrentToken() != JsonToken.END_OBJECT) {
p.nextToken();
field = p.getCurrentName();
obj.populateFromJsonString(field, p);
}
Output this:
while (p.getCurrentToken() != JsonToken.END_OBJECT) {
field = p.getCurrentName();
if (field!=null) {
obj.populateFromJsonString(field, p);
}
}
The -i.bak will edit in place and store a backup as filename.java.bak

How about this
$ searchvar=`cat searchblock.txt`
$ replvar=`cat replblock.txt`
$ sed -i ":loop; $! N; tloop; s/$searchvar/$replvar/g" *.java
where in searchblock.txt you'll put inputblock so on, with *.java you will catch all java files in folder
notice ` is not '
be careful while testing for -i inplace editing, you can try with backup file(s)

Execute *nix find command using java code [duplicate]

This question already has answers here:
Java execute command line program 'find' returns error
(2 answers)
Closed 10 years ago.
I am trying to execute a find command using java code. I did the following:
sysCommand = "find . -name '*out*' > file1"
Runtime runtimeObj = Runtime.getRuntime();
try {
Process processObj = runtimeObj.exec(sysCommand);
processObj.waitFor();
...
This Linux command is executed when I use command line but fails in Java, why?

As far as I know, it is not allowable to use any form of piping operator in Runtime.exec. If you want to move the results to a file, you will have to do that part in Java through Process.getInputStream.

If you are interested in doing this in Java then you will want to do something like this:
public void find(File startDirectory, FileFilter filter, List<File> matches) {
File[] files = startDirectory.listFiles(filter);
for (File file:files) {
if(file.isDirectory()) {
find(file, filter, matches);
} else {
matches.add(file);
}
}
}
Then you need but write the FileFilter to accept directories and files that match your pattern.

This question is probably a duplicate or a duplicate.
Anyway, you could use File.list, providing a Filter on the type
of files you want. You could call it recursively to get all sub-directories. I don't love this answer. You would think there is a simpler way.
A friend of mine recommended Commons-Exec from Apache for running a command. It allows you to use a time out on the command. He recommended it because Runtime can have issues with large stdout and stderr.

Launching java classes via windows drag-and-drop

I have a java class file with a main method. In Windows, I would like to be able to drag files onto a desktop icon/short/etc that would call supply the filenames to my main method. Basically, I want to allow users to drag-and-drop files at program execution instead of having type them on the command line.
Any thoughts?

To build on daub815's answer, in Windows, you can use a batch file to pass
arguments to another command. In this case, we'll use the java launcher to
launch your class with the main method.
I did a quick Google search on how to do write a batch file to take multiple arguments,
and found a page with a batch file to pass arguments to another command. Adapting from
the example, here is what you can do:
#ECHO OFF
:Loop
IF "%1" == "" GOTO Done
java YourClass %1
SHIFT
GOTO Loop
:Done
Save the above file as a batch file (with a ".bat" extension), and then you can drag-and-drop
files onto it, and it will be passed as arguments.
Also, you can call the batch file from the command line and pass arguments as well.
Edit: It appears that the batch file will not work with quoted arguments which contain spaces. Using a workaround presented in the site I've linked to will split the spaces contained in the quoted full path of the file into separate arguments, so that won't work either. If anyone has a good idea how to fix this, please either edit this entry, or post another answer. I will make this a community wiki.

PhiLho's answer works perfectly if you pack the classes in an executable JAR file (it's how you're meant to do it anyway) and make a .reg file that looks like the one below. Then just double-click that .reg file to merge it into the registry and you're good to go. This lets you both double-click a JAR file to run it, and starting it by Drag & Drop.
Do remember to change the path to where your Java executable is installed.
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.jar]
#="jarfile"
[HKEY_CLASSES_ROOT\jarfile\DefaultIcon]
#="C:\\Java\\jdk1.7.0\\bin\\java.exe,1"
[HKEY_CLASSES_ROOT\jarfile\shell\open]
#="Run Java Program"
[HKEY_CLASSES_ROOT\jarfile\shell\open\command]
#="\"C:\\Java\\jdk1.7.0\\bin\\java.exe\" -jar \"%1\" %*"
[HKEY_CLASSES_ROOT\jarfile\shellex\DropHandler]
#="{86C86720-42A0-1069-A2E8-08002B30309D}"

OK, I made it work... The base knowledge is to use DropHandler UUID in the registry. I made a base setting, as follow:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.class]
#="JavaClass"
[HKEY_CLASSES_ROOT\JavaClass\DefaultIcon]
#="C:\\Java\\jdk1.6.0_05\\bin\\java.exe,1"
[HKEY_CLASSES_ROOT\JavaClass\shell\open]
#="Run Java class"
[HKEY_CLASSES_ROOT\JavaClass\shell\open\command]
#="\"C:\\Java\\jdk1.6.0_05\\bin\\java.exe\" \"%1\" %*"
[HKEY_CLASSES_ROOT\JavaClass\shellex\DropHandler]
#="{86C86720-42A0-1069-A2E8-08002B30309D}"
and... it didn't work!
I just forgot that java.exe wants a class name, not a file name! But I see no way to do that in the registry.
Fortunately, there is a workaround, which still need a script file if we want to be generic, to work on any/all class files (with static main function, of course!). Not batch, I avoid them when I can. I chose to use WSH, as it should be available on any modern Windows system. I also chose to make a JS script, it could have been a VB script as well.
So I made the following script (LaunchJavaClass.js):
if (WScript.Arguments.count() == 0)
{
WScript.StdOut.Write("No parameters");
WScript.Quit(1);
}
var className = WScript.Arguments.Item(0);
//~ WScript.StdOut.Write(className + "\n");
var m = className.match(/^(.*)\\(.+?)\.class$/);
if (m == null)
{
WScript.StdOut.Write("Not a class file");
WScript.Quit(1);
}
var classPath = m[1];
className = m[2];
//~ WScript.StdOut.Write(classPath + " >>> " + className + "\n");
var params = new Array();
for (i = 1; i < WScript.Arguments.count(); i++)
{
params[params.length] = WScript.Arguments.Item(i);
}
var cmd = "cmd /c cd /D " + classPath +
" & C:/Java/jdk1.6.0_05/bin/java.exe " +
className + " " + params.join(" ");
//~ WScript.StdOut.Write(cmd + "\n");
var shell = WScript.CreateObject("WScript.Shell");
//~ var exec = shell.Exec(cmd); // Can be used to get stdout
shell.Run(cmd, 0);
I left some output, not useful in this context, but usable for debugging (run with cscript).
Of course, the path to the JRE must be adjusted.
And I changed the command in the registry, as follow:
[HKEY_CLASSES_ROOT\JavaClass\shell\open\command]
#="\wscript -b "D:\\_PhiLhoSoft\\WSH\\LaunchJavaClass.js\" %1 %*"
Of course, adjust path, and keep the above other lines.
Now, if I drag'n'drop some files to a .class file, it gets the short file paths as arguments of the main() function.
import java.io.*;
class TestDnD
{
public static void main(String[] args)
{
Writer output = null;
try
{
output = new BufferedWriter(new FileWriter(new File("LogFile.txt")));
for (String arg : args)
{
output.write(arg + "\n");
}
}
catch (IOException ioe)
{
ioe.printStackTrace();
return;
}
finally
{
try { output.close(); } catch (IOException e) {}
}
}
}
I think the first version of the .reg file can be used for something else, eg. to drag'n'drop on .jar files (adapting it, of course).
This technique has limited use: we rarely make one-class programs in Java! But it looked like a good and interesting challenge, so I didn't resist to solve it. Note: you can add stuff like -Djava.ext.dirs="some path;another path" if you ever need to use external libraries (in jar files).

Adding onto Adiel A. If you create a batch file, which launches your a Java window using Swing. You would have the user drop the files onto that window. You could then be able to root through those dropped files.

So there's no way to have windows itself pass the args into main() via drag and drop?

Is it possible to "add" to classpath dynamically in java?

java -classpath ../classes;../jar;. parserTester
How can i get the functionality in the above command programmatically? Like, is it possible to run as:
java parserTester
and get the same result? I tried using URLClassLoader but it modifies the classpath and does not add to it.
Thanx!
Thanks for the response Milhous. But that is what i am trying to do.. How is it possible to get the jar into the classpath first? I tried using a custom classloader too :(
That works.. But sorry that i need to run it only as:
java parserTester
I would like to know if such a thing is possible???
It needs to be so bcoz i have parserTester.java and .class in a separate folder. I need to retain the file structure. The parserTester makes use of a jar in a separate jar folder.

You can use a java.net.URLClassLoader to load classes with any program defined list of URL's you wish:
public class URLClassLoader
extends SecureClassLoader
This class loader is used to load
classes and resources from a search
path of URLs referring to both JAR
files and directories. Any URL that
ends with a '/' is assumed to refer to
a directory. Otherwise, the URL is
assumed to refer to a JAR file which
will be opened as needed.
The AccessControlContext of the thread
that created the instance of
URLClassLoader will be used when
subsequently loading classes and
resources.
The classes that are loaded are by
default granted permission only to
access the URLs specified when the
URLClassLoader was created.
Since:
1.2
And a little fancy footwork can extend it to support using wildcarded pathnames to pick up entire directories of JARs (this code has some references to utility methods, but their implementation should be obvious in the context):
/**
* Add classPath to this loader's classpath.
* <p>
* The classpath may contain elements that include a generic file base name. A generic basename
* is a filename without the extension that may begin and/or end with an asterisk. Use of the
* asterisk denotes a partial match. Any files with an extension of ".jar" whose base name match
* the specified basename will be added to this class loaders classpath. The case of the filename is ignored.
* For example "/somedir/*abc" means all files in somedir that end with "abc.jar", "/somedir/abc*"
* means all files that start with "abc" and end with ".jar", and "/somedir/*abc*" means all files
* that contain "abc" and end with ".jar".
*
*/
public void addClassPath(String cp) {
String seps=File.pathSeparator; // separators
if(!File.pathSeparator.equals(";")) { seps+=";"; } // want to accept both system separator and ';'
for(StringTokenizer st=new StringTokenizer(cp,seps,false); st.hasMoreTokens(); ) {
String pe=st.nextToken();
File fe;
String bn=null;
if(pe.length()==0) { continue; }
fe=new File(pe);
if(fe.getName().indexOf('*')!=-1) {
bn=fe.getName();
fe=fe.getParentFile();
}
if(!fe.isAbsolute() && pe.charAt(0)!='/' && pe.charAt(0)!='\\') { fe=new File(rootPath,fe.getPath()); }
try { fe=fe.getCanonicalFile(); }
catch(IOException thr) {
log.diagln("Skipping non-existent classpath element '"+fe+"' ("+thr+").");
continue;
}
if(!GenUtil.isBlank(bn)) {
fe=new File(fe,bn);
}
if(classPathElements.contains(fe.getPath())) {
log.diagln("Skipping duplicate classpath element '"+fe+"'.");
continue;
}
else {
classPathElements.add(fe.getPath());
}
if(!GenUtil.isBlank(bn)) {
addJars(fe.getParentFile(),bn);
}
else if(!fe.exists()) { // s/never be due getCanonicalFile() above
log.diagln("Could not find classpath element '"+fe+"'");
}
else if(fe.isDirectory()) {
addURL(createUrl(fe));
}
else if(fe.getName().toLowerCase().endsWith(".zip") || fe.getName().toLowerCase().endsWith(".jar")) {
addURL(createUrl(fe));
}
else {
log.diagln("ClassPath element '"+fe+"' is not an existing directory and is not a file ending with '.zip' or '.jar'");
}
}
log.diagln("Class loader is using classpath: \""+classPath+"\".");
}
/**
* Adds a set of JAR files using a generic base name to this loader's classpath. See #link:addClassPath(String) for
* details of the generic base name.
*/
public void addJars(File dir, String nam) {
String[] jars; // matching jar files
if(nam.endsWith(".jar")) { nam=nam.substring(0,(nam.length()-4)); }
if(!dir.exists()) {
log.diagln("Could not find directory for Class Path element '"+dir+File.separator+nam+".jar'");
return;
}
if(!dir.canRead()) {
log.error("Could not read directory for Class Path element '"+dir+File.separator+nam+".jar'");
return;
}
FileSelector fs=new FileSelector(true).add("BaseName","EG",nam,true).add("Name","EW",".jar",true);
if((jars=dir.list(fs))==null) {
log.error("Error accessing directory for Class Path element '"+dir+File.separator+nam+".jar'");
}
else if(jars.length==0) {
log.diagln("No JAR files match specification '"+new File(dir,nam)+".jar'");
}
else {
log.diagln("Adding files matching specification '"+dir+File.separator+nam+".jar'");
Arrays.sort(jars,String.CASE_INSENSITIVE_ORDER);
for(int xa=0; xa<jars.length; xa++) { addURL(createUrl(new File(dir,jars[xa]))); }
}
}
private URL createUrl(File fe) {
try {
URL url=fe.toURI().toURL();
log.diagln("Added URL: '"+url.toString()+"'");
if(classPath.length()>0) { classPath+=File.pathSeparator; }
this.classPath+=fe.getPath();
return url;
}
catch(MalformedURLException thr) {
log.diagln("Classpath element '"+fe+"' could not be used to create a valid file system URL");
return null;
}
}

I have to agree with the other two posters, it sounds like you're overcomplicating a test class.
It's not that unusual to have the .java and .class files in separate folders, while depending on jar files in yet a third, without programmatically changing the classpath.
If you're doing it because you don't want to have to type the classpath on the command line everytime, I would suggest a shell script or batch file. Better yet, an IDE.
The question I really have is why are you doing trying to manage the classpath in code?

You could implement your own class loader, but that class/jar has to be in the classpath for it to be executed.
try
java -cp *.jar:. myClass
or
export CLASSPATH=./lib/tool.jar:.
java myClass
or
java -jar file.jar

You can write a batch file or shell script file to export the classpath and run the java program.
In Windows,
set classpath=%classpath%;../classes;../jars/*
java ParserTester
In Unix,
export classpath=%classpath%:../classes:../jars/*
java ParserTester
If you name the file name as parser.bat or parser.sh, you can just run that by calling parser in respective OS.
From java 1.6, you can include all the jars in a directory into the classpath just by saying /*
If you are trying to generate a java file dynamically, compile and add into the classpath, set the directory into which the class file gets generated in the classpath beforehand. It should load the class.
If you are modifying the already generated java class, basically recompiling after modification and if you want to load the new class, you need to use your custom class loader to avoid the caching of the class.

I think what you want is an "Execution Wrapper" or a platform specific "Launcher"... typically this component is used to detect your OS and architecture and dependencies and then makes adjustments before launching your application. It is an old school design pattern (talking 80's and earlier) but is still used a lot today. The idea is that you program can be system and environment agnostic and the launcher will make preparations and tell the software everything it needs to know. Many modern open source programs do this with Shell scripts and Batch Files, etc... Apache Tomcat for example. You could just as easily make the wrapper in java an have it launch the software with a command line exec (be sure to add " &" to the end of you exec command in *NIX so your wrapper can exit leaving only your software running... also lets you close the shell window without killing the process)

Did I understand right?! The only reason you have it that you want to launch your class without specifying the classpath and load it at runtime? ...
java parserTester
instead of
java -classpath ../classes;../jar;. parserTester
Probably I didn't get your reason. But if "that's" what you want you can do the following ( although it does not make much sense to me )
Launch the class
From the main method lauch another class an programatically set the classpath there.
End of history.
Something like the following "java -pseudo code "
public static void main( String [] args ) {
String classpath = "classes;../jar";
Runtime.getRuntime().execute("java + classpath + " parserTester ");
}
Please tell me if I get it right. If you want to do something else I would gladly help.

Excellent good post, in my case I did this to work well (note: Windows specific):
set classpath=%classpath%;../lib/*
java -cp %classpath% com.test.MyClass