有三个库可以在单元测试期间做到这一点。

有Stefan Birkner的系统规则和系统Lambda - https://www.baeldung.com/java-system-rules-junit，允许你做一些事情:

public class JUnitTest {

    @Rule
    public EnvironmentVariables environmentVariables = new EnvironmentVariables();

    @Test
    public void someTest() {
        environmentVariables.set("SOME_VARIABLE", "myValue");
        
        // now System.getenv does what you want
    }
}

或System-Lambda:

@Test
void execute_code_with_environment_variables(
) throws Exception {
  List<String> values = withEnvironmentVariable("first", "first value")
    .and("second", "second value")
    .execute(() -> asList(
      System.getenv("first"),
      System.getenv("second")
    ));
  assertEquals(
    asList("first value", "second value"),
    values
  );
}

上述功能也可以通过系统存根作为JUnit 5扩展:

@ExtendWith(SystemStubsExtension.class)
class SomeTest {

    @SystemStub
    private EnvironmentVariables;

    @Test
    void theTest() {
        environmentVariables.set("SOME_VARIABLE", "myValue");
        
        // now System.getenv does what you want

    }

}

系统存根向后兼容System Lambda和System Rules，但支持JUnit 5。

另外，还有JUnit Pioneer - https://github.com/junit-pioneer/junit-pioneer，它允许在测试时通过注释设置环境变量。

尝试了上面pushy的答案，它在很大程度上起了作用。然而，在某些情况下，我会看到这个例外:

java.lang.String cannot be cast to java.lang.ProcessEnvironment$Variable

由于ProcessEnvironment的某些内部类的实现，当该方法被多次调用时就会发生这种情况。如果setEnv(..)方法被多次调用，当从theEnvironment映射中检索键时，它们现在是字符串(在第一次调用setEnv(…)时已作为字符串放入)，并且不能转换为映射的泛型类型Variable，这是ProcessEnvironment的私有内部类。

下面是一个固定版本(在Scala中)。希望将其应用到Java中并不太难。

def setEnv(newenv: java.util.Map[String, String]): Unit = {
  try {
    val processEnvironmentClass = JavaClass.forName("java.lang.ProcessEnvironment")
    val theEnvironmentField = processEnvironmentClass.getDeclaredField("theEnvironment")
    theEnvironmentField.setAccessible(true)

    val variableClass = JavaClass.forName("java.lang.ProcessEnvironment$Variable")
    val convertToVariable = variableClass.getMethod("valueOf", classOf[java.lang.String])
    convertToVariable.setAccessible(true)

    val valueClass = JavaClass.forName("java.lang.ProcessEnvironment$Value")
    val convertToValue = valueClass.getMethod("valueOf", classOf[java.lang.String])
    convertToValue.setAccessible(true)

    val sampleVariable = convertToVariable.invoke(null, "")
    val sampleValue = convertToValue.invoke(null, "")
    val env = theEnvironmentField.get(null).asInstanceOf[java.util.Map[sampleVariable.type, sampleValue.type]]
    newenv.foreach { case (k, v) => {
        val variable = convertToVariable.invoke(null, k).asInstanceOf[sampleVariable.type]
        val value = convertToValue.invoke(null, v).asInstanceOf[sampleValue.type]
        env.put(variable, value)
      }
    }

    val theCaseInsensitiveEnvironmentField = processEnvironmentClass.getDeclaredField("theCaseInsensitiveEnvironment")
    theCaseInsensitiveEnvironmentField.setAccessible(true)
    val cienv = theCaseInsensitiveEnvironmentField.get(null).asInstanceOf[java.util.Map[String, String]]
    cienv.putAll(newenv);
  }
  catch {
    case e : NoSuchFieldException => {
      try {
        val classes = classOf[java.util.Collections].getDeclaredClasses
        val env = System.getenv()
        classes foreach (cl => {
          if("java.util.Collections$UnmodifiableMap" == cl.getName) {
            val field = cl.getDeclaredField("m")
            field.setAccessible(true)
            val map = field.get(env).asInstanceOf[java.util.Map[String, String]]
            // map.clear() // Not sure why this was in the code. It means we need to set all required environment variables.
            map.putAll(newenv)
          }
        })
      } catch {
        case e2: Exception => e2.printStackTrace()
      }
    }
    case e1: Exception => e1.printStackTrace()
  }
}

事实证明，@pushy/@anonymous/@Edward Campbell的解决方案在Android上不起作用，因为Android并不是真正的Java。具体来说，Android根本没有java.lang.ProcessEnvironment。但事实证明在Android中更容易，你只需要对POSIX setenv()进行JNI调用:

打印C / JNI:

JNIEXPORT jint JNICALL Java_com_example_posixtest_Posix_setenv
  (JNIEnv* env, jclass clazz, jstring key, jstring value, jboolean overwrite)
{
    char* k = (char *) (*env)->GetStringUTFChars(env, key, NULL);
    char* v = (char *) (*env)->GetStringUTFChars(env, value, NULL);
    int err = setenv(k, v, overwrite);
    (*env)->ReleaseStringUTFChars(env, key, k);
    (*env)->ReleaseStringUTFChars(env, value, v);
    return err;
}

在Java中:

public class Posix {

    public static native int setenv(String key, String value, boolean overwrite);

    private void runTest() {
        Posix.setenv("LD_LIBRARY_PATH", "foo", true);
    }
}

这是@pushy的邪恶答案的Kotlin邪恶版本=)

@Suppress("UNCHECKED_CAST")
@Throws(Exception::class)
fun setEnv(newenv: Map<String, String>) {
    try {
        val processEnvironmentClass = Class.forName("java.lang.ProcessEnvironment")
        val theEnvironmentField = processEnvironmentClass.getDeclaredField("theEnvironment")
        theEnvironmentField.isAccessible = true
        val env = theEnvironmentField.get(null) as MutableMap<String, String>
        env.putAll(newenv)
        val theCaseInsensitiveEnvironmentField = processEnvironmentClass.getDeclaredField("theCaseInsensitiveEnvironment")
        theCaseInsensitiveEnvironmentField.isAccessible = true
        val cienv = theCaseInsensitiveEnvironmentField.get(null) as MutableMap<String, String>
        cienv.putAll(newenv)
    } catch (e: NoSuchFieldException) {
        val classes = Collections::class.java.getDeclaredClasses()
        val env = System.getenv()
        for (cl in classes) {
            if ("java.util.Collections\$UnmodifiableMap" == cl.getName()) {
                val field = cl.getDeclaredField("m")
                field.setAccessible(true)
                val obj = field.get(env)
                val map = obj as MutableMap<String, String>
                map.clear()
                map.putAll(newenv)
            }
        }
    }

它至少在macOS Mojave上运行。

像大多数找到这个线程的人一样，我正在编写一些单元测试，需要修改环境变量以设置测试运行的正确条件。然而，我发现被点赞最多的答案有一些问题和/或非常神秘或过于复杂。希望这能帮助其他人更快地找到解决方案。

首先，我终于发现@Hubert Grzeskowiak的解决方案是最简单的，对我来说很管用。我希望我能先讲到这一点。它基于@Edward Campbell的回答，但是没有复杂的for循环搜索。

然而，我从@pushy的解决方案开始，它得到了最多的赞。它是@anonymous和@Edward Campbell's的组合。@pushy声称这两种方法都需要覆盖Linux和Windows环境。我在OS X下运行，发现两者都可以工作(一旦@匿名方法的问题被修复)。正如其他人所指出的，这种解决方案在大多数情况下都有效，但并非所有情况都有效。

I think the source of most of the confusion comes from @anonymous's solution operating on the 'theEnvironment' field. Looking at the definition of the ProcessEnvironment structure, 'theEnvironment' is not a Map< String, String > but rather it is a Map< Variable, Value >. Clearing the map works fine, but the putAll operation rebuilds the map a Map< String, String >, which potentially causes problems when subsequent operations operate on the data structure using the normal API that expects Map< Variable, Value >. Also, accessing/removing individual elements is a problem. The solution is to access 'theEnvironment' indirectly through 'theUnmodifiableEnvironment'. But since this is a type UnmodifiableMap the access must be done through the private variable 'm' of the UnmodifiableMap type. See getModifiableEnvironmentMap2 in code below.

在我的例子中，我需要为我的测试删除一些环境变量(其他的应该保持不变)。然后，我想在测试之后将环境变量恢复到它们之前的状态。下面的例程会让你更容易做到。我在OS X上测试了两个版本的getModifiableEnvironmentMap，两者的工作效果相当。尽管基于本线程中的注释，但根据环境的不同，其中一种可能比另一种更好。

注意:我没有包括对' casaseinsensitiveenvironmentfield '的访问，因为这似乎是Windows特定的，我没有办法测试它，但添加它应该是直截了当地。

private Map<String, String> getModifiableEnvironmentMap() {
    try {
        Map<String,String> unmodifiableEnv = System.getenv();
        Class<?> cl = unmodifiableEnv.getClass();
        Field field = cl.getDeclaredField("m");
        field.setAccessible(true);
        Map<String,String> modifiableEnv = (Map<String,String>) field.get(unmodifiableEnv);
        return modifiableEnv;
    } catch(Exception e) {
        throw new RuntimeException("Unable to access writable environment variable map.");
    }
}

private Map<String, String> getModifiableEnvironmentMap2() {
    try {
        Class<?> processEnvironmentClass = Class.forName("java.lang.ProcessEnvironment");
        Field theUnmodifiableEnvironmentField = processEnvironmentClass.getDeclaredField("theUnmodifiableEnvironment");
        theUnmodifiableEnvironmentField.setAccessible(true);
        Map<String,String> theUnmodifiableEnvironment = (Map<String,String>)theUnmodifiableEnvironmentField.get(null);

        Class<?> theUnmodifiableEnvironmentClass = theUnmodifiableEnvironment.getClass();
        Field theModifiableEnvField = theUnmodifiableEnvironmentClass.getDeclaredField("m");
        theModifiableEnvField.setAccessible(true);
        Map<String,String> modifiableEnv = (Map<String,String>) theModifiableEnvField.get(theUnmodifiableEnvironment);
        return modifiableEnv;
    } catch(Exception e) {
        throw new RuntimeException("Unable to access writable environment variable map.");
    }
}

private Map<String, String> clearEnvironmentVars(String[] keys) {

    Map<String,String> modifiableEnv = getModifiableEnvironmentMap();

    HashMap<String, String> savedVals = new HashMap<String, String>();

    for(String k : keys) {
        String val = modifiableEnv.remove(k);
        if (val != null) { savedVals.put(k, val); }
    }
    return savedVals;
}

private void setEnvironmentVars(Map<String, String> varMap) {
    getModifiableEnvironmentMap().putAll(varMap);   
}

@Test
public void myTest() {
    String[] keys = { "key1", "key2", "key3" };
    Map<String, String> savedVars = clearEnvironmentVars(keys);

    // do test

    setEnvironmentVars(savedVars);
}

我偶然发现了这个线程，因为我有一个类似的需求，我需要设置(或更新)一个环境变量(永久地)。

所以我研究了如何从命令提示符永久设置一个环境变量，这是非常简单的!

setx JAVA_LOC C:/Java/JDK

然后我在代码中实现了同样的功能 这是我使用的(假设- JAVA_LOC是env。变量名)

        String cmdCommand = "setx JAVA_LOC " + "C:/Java/JDK";
            ProcessBuilder processBuilder = new ProcessBuilder();
            processBuilder.command("cmd.exe", "/c", cmdCommand);
            processBuilder.start();

ProcessBuilder启动cmd.exe并传递您想要的命令。 即使您关闭JVM/重新启动系统，环境变量也会保留，因为它与JVM/程序的生命周期没有关系。