Example sharing of how to use JPype to call java in python

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1. Brief introduction to JPype

1. What is JPype?

JPype is a tool that allows python code to easily call Java code, thus overcoming the shortcomings of python in some areas (such as server-side programming).

2. What is the difference between JPype and Jython (the successor of JPython)?

1) The running environment is different: jython runs on jvm, while the actual running environment of JPype is still python runtime, but an embedded jvm is started during operation;

2 ) users are different: jython is for java programs, and JPype is for python programmers.

2. JPype installation

1. Install Python2.7 and JAVA1.6 first

2. Install JPype- 0.5.4.2.win32-py2.7.exe(http://sourceforge.net/projects/jpype/files/JPype/0.5.4/)

3.Ubuntu12.04 installation command: sudo apt- get install python-jpype

3. JPype usage instructions

1. Start JVM

The startJVM() function provided by JPype is used to start the JAVA virtual machine, so before any subsequent JAVA code is called, this method must be called to start the JAVA virtual machine.

Definition of jpype.startJVM()

startJVM(jvm, *args)

Parameters of jpype.startJVM()

Parameter 1: jvm, describe the location of the jvm.dll file in your system The path, such as "C:\Program Files\IBM\Java50\jre\bin\j9vm\jvm.dll". The default JVM path can be obtained by calling jpype.getDefaultJVMPath().

Parameter 2: args, which is an optional parameter, will be passed directly to JVM by JPype as the startup parameter of the Java virtual machine. This is suitable for all legal JVM startup parameters, for example:

 -agentlib:libname[=options] 
 -classpath classpath 
 -verbose 
 -Xint

2. Shut down the JVM

When you are done using the JVM, you can shut it down through jpype.shutdownJVM() JVM, this function has no input parameters. When the python program exits, the JVM will automatically shut down.

3. Reference third-party Java extension packages

Many times, third-party Java extension packages need to be called in python projects, which is also an important use of JPype.

By adding: -Djava.class.path=ext_classpath to the JVM startup parameters, you can call existing Java extension packages in python code.

4. Access JAVA system properties

Sometimes, some Java applications need to set or get system properties in the JVM.

1. Example of setting system variables when JVM starts:

2. Add the following parameters to the JVM startup parameters:

-Dproperty=value

4. Example

1. Directly call JAVA API

from jpype import * 
import os.path 
startJVM("C:/Java/jdk1.6.0_10/jre/bin/client/jvm.dll", "-ea") 
java.lang.System.out.println("hello World") 
shutdownJVM()

2. Call JAVA third-party extension package

1) JAVA custom third-party jar package: package the JpypeDemo class as a jpypedemo.jar file and store it in the F:/sample_Py directory

package jpype; 
 
public class JpypeDemo { 
public String sayHello(String user){ 
return "hello" + user; 
} 
public int calc(int a, int b){ 
return a + b;  
} 
}

2) Python calls the third-party JAVA jar package program

from jpype import * 
import os.path 
jarpath = os.path.join(os.path.abspath('.'), 'F:/sample_Py/') 
startJVM("C:/Java/jdk1.6.0_10/jre/bin/client/jvm.dll","-ea", "-Djava.class.path=%s" % (jarpath + 'jpypedemo.jar')) 
#ubuntu 中startJVM("/home/geek/Android/jdk1.6.0_43/jre/lib/i386/server/libjvm.so","-ea", "-Djava.class.path=%s" % (jarpath + 'XXX.jar')) 
JDClass = JClass("jpype.JpypeDemo") 
jd = JDClass() 
#jd = JPackage("jpype").JpypeDemo() #两种创建jd的方法 
jprint = java.lang.System.out.println 
jprint(jd.sayHello("waw")) 
jprint(jd.calc(2,4)) 
shutdownJVM()

3. Access JAVA system properties

Suppose the property you want to set is named yourProperty and the property value is yourValue.

1) Example of setting system variables when JVM starts

import jpype 
 jvmPath = jpype.getDefaultJVMPath() 
 jvmArg = “ -DyourProperty=yourValue ”
 if not jpype.isJVMStarted(): 
  jpype.startJVM(jvmPath，jvmArg)

2) Example of setting system variables in the program

import jpype 
 prop = “ yourProperty ”
 value = “ yourValue ”
 system = jpype.JClass('java.lang.System') 
 system.setProperty(str(prop),str(value))

3) Example of getting system variables in the program

import jpype 
 prop = “ yourProperty ”
 system = jpype.JClass('java.lang.System') 
 value = system.getProperty(str(prop))

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