Replacing system environment variables by reflection in java

When writing test code with jUnit etc.

System.You may want to temporarily replace the value of the system environment variable that you get with getenv.

 I will describe two implementations, one that does not use the library and the other that uses PowerMock.

 Dependency
Java 8 
 * I think it will work with Java 7.

# Implementation without library
 The system environment variable ``` TMEP``` has been replaced.

```java
import org.junit.Test;

import java.lang.reflect.Modifier;
import java.lang.reflect.Field;
import java.util.Map;

import static org.junit.Assert.assertEquals;

public class EnvironmentTest {
  @Test
  public void replaceEnvironment() throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException {
	//Get Final Class methods and make them accessible
	Class<?> clazz = Class.forName("java.lang.ProcessEnvironment");
    Field theCaseInsensitiveEnvironment = clazz.getDeclaredField("theCaseInsensitiveEnvironment");
	theCaseInsensitiveEnvironment.setAccessible(true);

	//Replace only the system environment variables you need
	Map<String,String> sytemEnviroment = (Map<String, String>) theCaseInsensitiveEnvironment.get(null);
    sytemEnviroment.put("TEMP","/MY_TEMP_DIR");

	assertEquals(System.getenv("TEMP"),"/MY_TEMP_DIR");
  }
}

PowerMock

PowerMock is relatively easy to implement. However, Class needs to be annotated.

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import static org.junit.Assert.assertEquals;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ System.class })
public class EnvironmentWithPowerMockTest {
  @Test
  public void replaceEnvironment(){
	PowerMockito.mockStatic(System.class);
	PowerMockito.when(System.getenv("TEMP")).thenReturn("/MY_TEMP_DIR");

	assertEquals(System.getenv("TEMP"),"/MY_TEMP_DIR");
  }
}

Remarks

The sample source code is below. https://github.com/amanoese/java-reflection-sample

The source code of System Rules was similar, so basically this is okay. https://github.com/stefanbirkner/system-rules/blob/master/src/main/java/org/junit/contrib/java/lang/system/EnvironmentVariables.java#L165-L190

reference

https://qiita.com/5at00001040/items/83bd7ea85d0f545ae7c3 https://blogs.yahoo.co.jp/dk521123/37484564.html https://stackoverflow.com/questions/318239/how-do-i-set-environment-variables-from-java