Introduction
In the GhostPDL repository sample Java projects can be found in /demos/java.
Within this location the following folders are of relevance:
Platform & setup
Ghostscript should be built as a shared library for your platform.
See Building Ghostscript.
jni: Building the Java Native Interface
Before building the JNI ensure that Ghostscript has already been built for your platform and that you have JDK installed.
The JNI is for use in the Java interface, this object must be placed somewhere on your Java PATH. On Windows, the DLL can be placed in the working directory, next to gsjava.jar.
Platform |
JNI file |
|---|---|
Windows |
|
MacOS |
|
Linux / OpenBSD |
|
Preparing your include folder
The build scripts require the header jni.h, which defines all JNI functions, and jni_md.h, which defines all system-specific integer types. The build scripts expect an include folder relative to their location which contain these header files from your system.
These headers are typically found in the following directories:
Platform |
jni.h |
jni_md.h |
|---|---|---|
Windows |
C:Program FilesJava<JDK Install>includejni.h |
C:Program FilesJava<JDK Install>includewin32jni_md.h |
MacOS |
/Library/Java/JavaVirtualMachines/<JDK Install>/Contents/Home/include/jni.h |
/Library/Java/JavaVirtualMachines/<JDK Install>/Contents/Home/include/darwin/jni_md.h |
Linux |
/lib/jvm/<JDK Install>/include/jni.h |
/lib/jvm/<JDK Install>/include/linux/jni_md.h |
Once your include folder has been located folder you can copy it and place it in your ghostpdl/demos/java/jni/gs_jni folder.
Your build scripts should now be ready to run as they will be able to find the required JNI header files in their own relative include folder.
Building on Windows
The jni folder contains a Visual Studio Solution file /jni/gs_jni/gs_jni.sln which you should use to build the required JNI gs_jni.dll library file.
With the project open in Visual Studio, select the required architecture from the drop down - then right click on ‘gs_jni’ in the solution explorer and choose “Build”.
Building on MacOS
On your command line, navigate to ghostpdl/demos/java/jni/gs_jni and ensure that the build script is executable and then run it, with:
chmod +x build_darwin.sh
./build_darwin.sh
Building on Linux
On your command line, navigate to ghostpdl/demos/java/jni/gs_jni and ensure that the build script is executable and then run it, with:
chmod +x build_linux.sh
./build_linux.sh
gsjava: Building the JAR
gsjava.jar is the Java library which contains classes and interfaces which enable API calls required to use Ghostscript.
Assuming that the JAR for your project has been built and properly linked with your own project then the Ghostscript API should be available by importing the required classes within your project’s .java files.
Building with the command line
Navigate to ghostpdl/demos/java/gsjava and use the following:
Platform |
Run file |
|---|---|
Windows |
build_win32.bat |
MacOS |
build_darwin.sh |
Linux |
build_linux.sh |
Note
gsjava has a dependency on jni, please ensure that gs_jni is able to be built beforehand.
Building with Eclipse
Alternatively you can use Eclipse to build the JAR file.
Using Eclipse import the source folder gsjava as a project and select Export > Java > JAR File as shown in the screenshot example below:
Linking the JAR
The built JAR should be properly linked within your project Java Build Path as follows:
Demo projects
gstest
Use this to quickly test Ghostscript methods.
This project can be opened in Eclipse and used to test the Ghostscript API. The sample here simply sets up an instance of Ghostscript and then sets and gets some parameters accordingly.
gsviewer
A handy file viewer.
This project can be used to test the Ghostscript API alongside a UI which handles opening PostScript and PDF files. The sample application here allows for file browsing and Ghostscript file viewing.
Below is a screenshot of the sample application with a PDF open:
To run the project navigate to the demos/java/gsviewer location and ensure that the required libraries are in the directory:
Platform |
Ghostscript library file |
JNI library file |
|---|---|---|
Windows |
|
|
MacOS |
|
|
Linux / OpenBSD |
so it should be copied into this directory and renamed to |
|
Building on Windows
Run the build_win32.bat script.
Running on Windows
To run, open gsviewer.jar either through File Explorer or in the command line through the following command:
java -jar gsviewer.jar
Building on MacOS
On your command line, navigate to ghostpdl/demos/java/gsviewer and ensure that the build script is executable and then run it, with:
chmod +x build_darwin.sh
./build_darwin.sh
This will automatically build gs_jni.dylib (in the ghostpdl/demos/java/jni/gs_jni/ location) and gsjava.jar gsviewer.jar in the gsviewer directory.
Running on MacOS
Ensure that the Ghostscript library exists in the gsviewer directory. (Copy and move the built library from ghostpdl/sobin as required).
Ensure that the run script is executable and then run it, with:
chmod +x start_darwin.sh
./start_darwin.sh
Building on Linux
On your command line, navigate to ghostpdl/demos/java/gsviewer and ensure that the build script is executable and then run it, with:
chmod +x build_linux.sh
./build_linux.sh
This will automatically build gs_jni.so (in the ghostpdl/demos/java/jni/gs_jni/ location) and gsjava.jar gsviewer.jar in the gsviewer directory.
Note
On Linux, when using OpenJDK, the property “assistive_technologies” may need to be modified for the Java code to build. It can be modified by editing the “accessibility.properties” file. This is located at:
/etc/java-8-openjdk/accessibility.properties
Running on Linux
Ensure that the Ghostscript library exists in the gsviewer directory. (Copy and move the built library from ghostpdl/sobin as required).
Ensure that the run script is executable and then run it, with:
chmod +x start_linux.sh
./start_linux.sh
Using the Java library
gsjava
There are two main classes within the gsjava.jar library to consider:
GSAPI & GSInstance
GSAPI is the main Ghostscript API class which bridges into the Ghostscript C library.
GSInstance is a wrapper class for GSAPI which encapsulates an instance of Ghostscript and allows for simpler API calls.
Sample code:
// to use GSAPI
import static com.artifex.gsjava.GSAPI.*;
// to use GSInstance
import com.artifex.gsjava.GSInstance;
GSAPI
gsapi_revision
This method returns the revision numbers and strings of the Ghostscript interpreter library; you should call it before any other interpreter library functions to make sure that the correct version of the Ghostscript interpreter has been loaded.
public static native int gsapi_revision(GSAPI.Revision revision,
int len);
Note
The method should write to a reference variable which conforms to the class GSAPI.Revision.
GSAPI.Revision
This class is used to store information about Ghostscript and provides handy getters for the product and the copyright information.
public static class Revision {
public volatile byte[] product;
public volatile byte[] copyright;
public volatile long revision;
public volatile long revisionDate;
public Revision() {
this.product = null;
this.copyright = null;
this.revision = 0L;
this.revisionDate = 0L;
}
/**
* Returns the product information as a String.
*
* @return The product information.
*/
public String getProduct() {
return new String(product);
}
/**
* Returns the copyright information as a String.
*
* @return The copyright information.
*/
public String getCopyright() {
return new String(copyright);
}
}
gsapi_new_instance
Creates a new instance of Ghostscript. This instance is passed to most other GSAPI methods. Unless Ghostscript has been compiled with the GS_THREADSAFE define, only one instance at a time is supported.
public static native int gsapi_new_instance(Reference<Long> instance,
long callerHandle);
Note
The method returns a reference which represents your instance of Ghostscript.
gsapi_delete_instance
Destroy an instance of Ghostscript. Before you call this, Ghostscript must have finished. If Ghostscript has been initialised, you should call gsapi_exit beforehand.
public static native void gsapi_delete_instance(long instance);
gsapi_set_stdio_with_handle
Set the callback functions for stdio, together with the handle to use in the callback functions. The stdin callback function should return the number of characters read, 0 for EOF, or -1 for error. The stdout and stderr callback functions should return the number of characters written.
public static native int gsapi_set_stdio_with_handle(long instance,
IStdInFunction stdin,
IStdOutFunction stdout,
IStdErrFunction stderr,
long callerHandle);
gsapi_set_stdio
Set the callback functions for stdio. The handle used in the callbacks will be taken from the value passed to gsapi_new_instance. Otherwise the behaviour of this function matches gsapi_set_stdio_with_handle.
public static native int gsapi_set_stdio(long instance,
IStdInFunction stdin,
IStdOutFunction stdout,
IStdErrFunction stderr);
gsapi_set_poll_with_handle
Set the callback function for polling, together with the handle to pass to the callback function. This function will only be called if the Ghostscript interpreter was compiled with CHECK_INTERRUPTS as described in gpcheck.h.
The polling function should return zero if all is well, and return negative if it wants Ghostscript to abort. This is often used for checking for a user cancel. This can also be used for handling window events or cooperative multitasking.
The polling function is called very frequently during interpretation and rendering so it must be fast. If the function is slow, then using a counter to return 0 immediately some number of times can be used to reduce the performance impact.
public static native int gsapi_set_poll_with_handle(long instance,
IPollFunction pollfun,
long callerHandle);
gsapi_set_poll
Set the callback function for polling. The handle passed to the callback function will be taken from the handle passed to gsapi_new_instance. Otherwise the behaviour of this function matches gsapi_set_poll_with_handle.
public static native int gsapi_set_poll(long instance,
IPollFunction pollfun);
gsapi_set_display_callback
This call is deprecated; please use gsapi_register_callout to register a callout handler for the display device in preference.
public static native int gsapi_set_display_callback(long instance,
DisplayCallback displayCallback);
gsapi_register_callout
This call registers a callout handler.
public static native int gsapi_register_callout(long instance,
ICalloutFunction callout,
long calloutHandle);
gsapi_deregister_callout
This call deregisters a callout handler previously registered with gsapi_register_callout. All three arguments must match exactly for the callout handler to be deregistered.
public static native void gsapi_deregister_callout(long instance,
ICalloutFunction callout,
long calloutHandle);
gsapi_set_arg_encoding
Set the encoding used for the interpretation of all subsequent arguments supplied via the GSAPI interface on this instance. By default we expect args to be in encoding 0 (the ‘local’ encoding for this OS). On Windows this means “the currently selected codepage”. This means that omitting to call this function will leave Ghostscript running exactly as it always has. Please note that use of the ‘local’ encoding is now deprecated and should be avoided in new code. This must be called after gsapi_new_instance and before gsapi_init_with_args.
public static native int gsapi_set_arg_encoding(long instance,
int encoding);
gsapi_set_default_device_list
Set the string containing the list of default device names, for example “display x11alpha x11 bbox”. Allows the calling application to influence which device(s) Ghostscript will try, in order, in its selection of the default device. This must be called after gsapi_new_instance and before gsapi_init_with_args.
public static native int gsapi_set_default_device_list(long instance,
byte[] list,
int listlen);
gsapi_get_default_device_list
Returns a pointer to the current default device string. This must be called after gsapi_new_instance and before gsapi_init_with_args.
public static native int gsapi_get_default_device_list(long instance,
Reference<byte[]> list,
Reference<Integer> listlen);
gsapi_init_with_args
To initialise the interpreter, pass your instance of Ghostscript, your argument count: argc, and your argument variables: argv.
public static native int gsapi_init_with_args(long instance,
int argc,
byte[][] argv);
Note
There are also simpler utility methods which eliminates the need to send through your argument count and allows for simpler String passing for your argument array.
Utility methods:
public static int gsapi_init_with_args(long instance,
String[] argv);
public static int gsapi_init_with_args(long instance,
List<String> argv);
gsapi_run_*
If these functions return <= -100, either quit or a fatal error has occured. You must call java gsapi_exit next. The only exception is gsapi_run_string_continue which will return gs_error_NeedInput if all is well.
There is a 64 KB length limit on any buffer submitted to a gsapi_run_* function for processing. If you have more than 65535 bytes of input then you must split it into smaller pieces and submit each in a separate gsapi_run_string_continue call.
gsapi_run_string_begin
public static native int gsapi_run_string_begin(long instance,
int userErrors,
Reference<Integer> pExitCode);
gsapi_run_string_continue
public static native int gsapi_run_string_continue(long instance,
byte[] str,
int length,
int userErrors,
Reference<Integer> pExitCode);
Note
There is a simpler utility method which allows for simpler String passing for the str argument.
Utility method:
public static int gsapi_run_string_continue(long instance,
String str,
int length,
int userErrors,
Reference<Integer> pExitCode);
gsapi_run_string_with_length
public static native int gsapi_run_string_with_length(long instance,
byte[] str,
int length,
int userErrors,
Reference<Integer> pExitCode);
Note
There is a simpler utility method which allows for simpler String passing for the str argument.
Utility method:
public static int gsapi_run_string_with_length(long instance,
String str,
int length,
int userErrors,
Reference<Integer> pExitCode);
gsapi_run_string
public static native int gsapi_run_string(long instance,
byte[] str,
int userErrors,
Reference<Integer> pExitCode);
Note
There is a simpler utility method which allows for simpler String passing for the str argument.
Utility method:
public static int gsapi_run_string(long instance,
String str,
int userErrors,
Reference<Integer> pExitCode);
gsapi_run_string_end
public static native int gsapi_run_string_end(long instance,
int userErrors,
Reference<Integer> pExitCode);
gsapi_run_file
public static native int gsapi_run_file(long instance,
byte[] fileName,
int userErrors,
Reference<Integer> pExitCode);
Note
There is a simpler utility method which allows for simpler String passing for the fileName argument.
Utility method:
public static int gsapi_run_file(long instance,
String fileName,
int userErrors,
Reference<Integer> pExitCode);
gsapi_exit
Exit the interpreter. This must be called on shutdown if gsapi_init_with_args has been called, and just before gsapi_delete_instance.
public static native int gsapi_exit(long instance);
gsapi_set_param
Sets a parameter. Broadly, this is equivalent to setting a parameter using -d, -s or -p on the command line. This call cannot be made during a gsapi_run_string operation.
Parameters in this context are not the same as ‘arguments’ as processed by gsapi_init_with_args, but often the same thing can be achieved. For example, with gsapi_init_with_args, we can pass “-r200” to change the resolution. Broadly the same thing can be achieved by using gsapi_set_param to set a parsed value of “<</HWResolution [ 200.0 200.0 ]>>”.
Internally, when we set a parameter, we perform an initgraphics operation. This means that using gsapi_set_param other than at the start of a page is likely to give unexpected results.
Attempting to set a parameter that the device does not recognise will be silently ignored, and that parameter will not be found in subsequent gsapi_get_param calls.
public static native int gsapi_set_param(long instance,
byte[] param,
Object value,
int paramType);
Note
For more on the C implementation of parameters see: Ghostscript parameters in C.
There are also simpler utility methods which allows for simpler String passing for your arguments.
Utility methods:
public static int gsapi_set_param(long instance,
String param,
String value,
int paramType);
public static int gsapi_set_param(long instance,
String param,
Object value,
int paramType);
gsapi_get_param
Retrieve the current value of a parameter.
If an error occurs, the return value is negative. Otherwise the return value is the number of bytes required for storage of the value. Call once with value NULL to get the number of bytes required, then call again with value pointing to at least the required number of bytes where the value will be copied out. Note that the caller is required to know the type of value in order to get it. For all types other than gs_spt_string, gs_spt_name, and gs_spt_parsed knowing the type means you already know the size required.
This call retrieves parameters/values that have made it to the device. Thus, any values set using gs_spt_more_to_come without a following call omitting that flag will not be retrieved. Similarly, attempting to get a parameter before gsapi_init_with_args has been called will not list any, even if gsapi_set_param has been used.
Attempting to read a parameter that is not set will return gs_error_undefined (-21). Note that calling gsapi_set_param followed by gsapi_get_param may not find the value, if the device did not recognise the key as being one of its configuration keys.
For further documentation please refer to the C API.
public static native int gsapi_get_param(long instance,
byte[] param,
long value,
int paramType);
Note
There is a simpler utility method which allows for simpler String passing for the param argument.
Utility method:
public static int gsapi_get_param(long instance,
String param,
long value,
int paramType);
gsapi_enumerate_params
Enumerate the current parameters. Call repeatedly to list out the current parameters.
The first call should have iter = NULL. Subsequent calls should pass the same pointer in so the iterator can be updated. Negative return codes indicate error, 0 success, and 1 indicates that there are no more keys to read. On success, key will be updated to point to a null terminated string with the key name that is guaranteed to be valid until the next call to gsapi_enumerate_params. If type is non NULL then the pointer type will be updated to have the type of the parameter.
Note
Only one enumeration can happen at a time. Starting a second enumeration will reset the first.
The enumeration only returns parameters/values that have made it to the device. Thus, any values set using the gs_spt_more_to_come without a following call omitting that flag will not be retrieved. Similarly, attempting to enumerate parameters before gsapi_init_with_args has been called will not list any, even if gsapi_set_param has been used.
public static native int gsapi_enumerate_params(long instance,
Reference<Long> iter,
Reference<byte[]> key,
Reference<Integer> paramType);
gsapi_add_control_path
Add a (case sensitive) path to one of the lists of permitted paths for file access.
public static native int gsapi_add_control_path(long instance,
int type,
byte[] path);
Note
There is a simpler utility method which allows for simpler String passing for the path argument.
Utility method:
public static int gsapi_add_control_path(long instance,
int type,
String path);
gsapi_remove_control_path
Remove a (case sensitive) path from one of the lists of permitted paths for file access.
public static native int gsapi_remove_control_path(long instance,
int type,
byte[] path);
Note
There is a simpler utility method which allows for simpler String passing for the path argument.
Utility method:
public static int gsapi_remove_control_path(long instance,
int type,
String path);
gsapi_purge_control_paths
Clear all the paths from one of the lists of permitted paths for file access.
public static native void gsapi_purge_control_paths(long instance,
int type);
gsapi_activate_path_control
Enable/Disable path control (i.e. whether paths are checked against permitted paths before access is granted).
public static native void gsapi_activate_path_control(long instance,
boolean enable);
gsapi_is_path_control_active
Query whether path control is activated or not.
public static native boolean gsapi_is_path_control_active(long instance);
Callback & Callout interfaces
gsjava.jar also defines some functional interfaces for callbacks & callouts with package com.artifex.gsjava.callback which are defined as follows.
IStdInFunction
public interface IStdInFunction {
/**
* @param callerHandle The caller handle.
* @param buf A string represented by a byte array.
* @param len The number of bytes to read.
* @return The number of bytes read, must be <code>len</code>/
*/
public int onStdIn(long callerHandle,
byte[] buf,
int len);
}
IStdOutFunction
public interface IStdOutFunction {
/**
* Called when something should be written to the standard
* output stream.
*
* @param callerHandle The caller handle.
* @param str The string represented by a byte array to write.
* @param len The number of bytes to write.
* @return The number of bytes written, must be <code>len</code>.
*/
public int onStdOut(long callerHandle,
byte[] str,
int len);
}
IStdErrFunction
public interface IStdErrFunction {
/**
* Called when something should be written to the standard error stream.
*
* @param callerHandle The caller handle.
* @param str The string represented by a byte array to write.
* @param len The length of bytes to be written.
* @return The amount of bytes written, must be <code>len</code>.
*/
public int onStdErr(long callerHandle,
byte[] str,
int len);
}
IPollFunction
public interface IPollFunction {
public int onPoll(long callerHandle);
}
ICalloutFunction
public interface ICalloutFunction {
public int onCallout(long instance,
long calloutHandle,
byte[] deviceName,
int id,
int size,
long data);
}
GSInstance
This is a utility class which makes Ghostscript calls easier by storing a Ghostscript instance and, optionally, a caller handle. Essentially the class acts as a handy wrapper for the standard GSAPI methods.
Constructors
public GSInstance() throws IllegalStateException;
public GSInstance(long callerHandle) throws IllegalStateException;
delete_instance
Wraps gsapi_delete_instance.
public void delete_instance();
set_stdio
Wraps gsapi_set_stdio.
public int set_stdio(IStdInFunction stdin,
IStdOutFunction stdout,
IStdErrFunction stderr);
set_poll
Wraps gsapi_set_poll.
public int set_poll(IPollFunction pollfun);
set_display_callback
Wraps gsapi_set_display_callback.
public int set_display_callback(DisplayCallback displaycallback);
register_callout
Wraps gsapi_register_callout.
public int register_callout(ICalloutFunction callout);
deregister_callout
Wraps gsapi_deregister_callout.
public void deregister_callout(ICalloutFunction callout);
set_arg_encoding
Wraps gsapi_set_arg_encoding.
public int set_arg_encoding(int encoding);
set_default_device_list
Wraps gsapi_set_default_device_list.
public int set_default_device_list(byte[] list,
int listlen);
get_default_device_list
Wraps gsapi_get_default_device_list.
public int get_default_device_list(Reference<byte[]> list,
Reference<Integer> listlen);
init_with_args
Wraps gsapi_init_with_args.
public int init_with_args(int argc,
byte[][] argv);
public int init_with_args(String[] argv);
public int init_with_args(List<String> argv);
run_string_begin
Wraps gsapi_run_string_begin.
public int run_string_begin(int userErrors,
Reference<Integer> pExitCode);
run_string_continue
Wraps gsapi_run_string_continue.
public int run_string_continue(byte[] str,
int length,
int userErrors,
Reference<Integer> pExitCode);
public int run_string_continue(String str,
int length,
int userErrors,
Reference<Integer> pExitCode);
run_string
Wraps gsapi_run_string.
public int run_string(byte[] str,
int userErrors,
Reference<Integer> pExitCode);
public int run_string(String str,
int userErrors,
Reference<Integer> pExitCode);
run_file
Wraps gsapi_run_file.
public int run_file(byte[] fileName,
int userErrors,
Reference<Integer> pExitCode);
public int run_file(String filename,
int userErrors,
Reference<Integer> pExitCode);
exit
Wraps gsapi_exit.
public int exit();
set_param
Wraps gsapi_set_param.
public int set_param(byte[] param,
Object value,
int paramType);
public int set_param(String param,
String value,
int paramType);
public int set_param(String param,
Object value,
int paramType);
get_param
Wraps gsapi_get_param.
public int get_param(byte[] param,
long value,
int paramType);
public int get_param(String param,
long value,
int paramType);
enumerate_params
Wraps gsapi_enumerate_params.
public int enumerate_params(Reference<Long> iter,
Reference<byte[]> key,
Reference<Integer> paramType);
add_control_path
Wraps gsapi_add_control_path.
public int add_control_path(int type,
byte[] path);
public int add_control_path(int type,
String path);
remove_control_path
Wraps gsapi_remove_control_path.
public int remove_control_path(int type,
byte[] path);
public int remove_control_path(int type,
String path);
purge_control_paths
Wraps gsapi_purge_control_paths.
public void purge_control_paths(int type);
activate_path_control
Wraps gsapi_activate_path_control.
public void activate_path_control(boolean enable);
is_path_control_active
Wraps gsapi_is_path_control_active.
public boolean is_path_control_active();
Utility classes
The com.artifex.gsjava.util package contains a set of classes and interfaces which are used throughout the API.
com.artifex.gsjava.util.Reference
Reference<T> is used in many of the Ghostscript calls, it stores a reference to a generic value of type T. This class exists to emulate pointers being passed to a native function. Its value can be fetched with getValue() and set with setValue(T value).
public class Reference<T> {
private volatile T value;
public Reference() {
this(null);
}
public Reference(T value) {
this.value = value;
}
public void setValue(T value) {
this.value = value;
}
public T getValue() {
return value;
}
...
}