如何从Java中设置环境变量?

你可以使用-D将参数传递到你的初始java进程:

java -cp <classpath> -Dkey1=value -Dkey2=value ...

(是不是因为这是Java，所以我不应该做邪恶的、不可移植的、过时的事情，比如触摸我的环境?)

我认为你说到点子上了。

减轻负担的一个可能的方法是找出一种方法

void setUpEnvironment(ProcessBuilder builder) {
Map<String, String> env = builder.environment();
// blah blah
}

并在启动它们之前传递任何__abc。

另外，你可能已经知道这一点，但是你可以用相同的ProcessBuilder启动多个进程。因此，如果子流程是相同的，就不需要一遍又一遍地进行这个设置。

在网上闲逛，似乎可以用JNI来实现这一点。然后，您必须从C调用putenv()，并且(大概)必须以在Windows和UNIX上都能运行的方式来执行此操作。

如果所有这些都可以做到，那么对于Java本身来说，支持这一点，而不是让我穿上紧身夹克，肯定不会太难。

在其他地方讲perl的朋友认为，这是因为环境变量是进程全局的，而Java正在努力为良好的设计提供良好的隔离。

public static void set(Map<String, String> newenv) throws Exception {
Class[] classes = Collections.class.getDeclaredClasses();
Map<String, String> env = System.getenv();
for(Class cl : classes) {
if("java.util.Collections$UnmodifiableMap".equals(cl.getName())) {
Field field = cl.getDeclaredField("m");
field.setAccessible(true);
Object obj = field.get(env);
Map<String, String> map = (Map<String, String>) obj;
map.clear();
map.putAll(newenv);
}
}
}

或者根据joshwolfe的建议添加/更新一个单独的var并删除循环。

@SuppressWarnings({ "unchecked" })
public static void updateEnv(String name, String val) throws ReflectiveOperationException {
Map<String, String> env = System.getenv();
Field field = env.getClass().getDeclaredField("m");
field.setAccessible(true);
((Map<String, String>) field.get(env)).put(name, val);
}

// this is a dirty hack - but should be ok for a unittest.
private void setNewEnvironmentHack(Map<String, String> newenv) throws Exception
{
Class<?> processEnvironmentClass = Class.forName("java.lang.ProcessEnvironment");
Field theEnvironmentField = processEnvironmentClass.getDeclaredField("theEnvironment");
theEnvironmentField.setAccessible(true);
Map<String, String> env = (Map<String, String>) theEnvironmentField.get(null);
env.clear();
env.putAll(newenv);
Field theCaseInsensitiveEnvironmentField = processEnvironmentClass.getDeclaredField("theCaseInsensitiveEnvironment");
theCaseInsensitiveEnvironmentField.setAccessible(true);
Map<String, String> cienv = (Map<String, String>) theCaseInsensitiveEnvironmentField.get(null);
cienv.clear();
cienv.putAll(newenv);
}

在需要为单元测试设置特定环境值的场景中使用，您可能会发现以下hack很有用。它将在JVM中更改环境变量(因此请确保在测试后重置所有更改)，但不会更改系统环境。

我发现Edward Campbell和anonymous的两个肮脏的黑客组合效果最好，因为一个在linux下不起作用，另一个在windows 7下也不起作用。所以为了获得一个多平台的邪恶黑客，我将它们结合起来:

protected static void setEnv(Map<String, String> newenv) throws Exception {
try {
Class<?> processEnvironmentClass = Class.forName("java.lang.ProcessEnvironment");
Field theEnvironmentField = processEnvironmentClass.getDeclaredField("theEnvironment");
theEnvironmentField.setAccessible(true);
Map<String, String> env = (Map<String, String>) theEnvironmentField.get(null);
env.putAll(newenv);
Field theCaseInsensitiveEnvironmentField = processEnvironmentClass.getDeclaredField("theCaseInsensitiveEnvironment");
theCaseInsensitiveEnvironmentField.setAccessible(true);
Map<String, String> cienv = (Map<String, String>) theCaseInsensitiveEnvironmentField.get(null);
cienv.putAll(newenv);
} catch (NoSuchFieldException e) {
Class[] classes = Collections.class.getDeclaredClasses();
Map<String, String> env = System.getenv();
for(Class cl : classes) {
if("java.util.Collections$UnmodifiableMap".equals(cl.getName())) {
Field field = cl.getDeclaredField("m");
field.setAccessible(true);
Object obj = field.get(env);
Map<String, String> map = (Map<String, String>) obj;
map.clear();
map.putAll(newenv);
}
}
}
}

这招很管用。完全归功于这些黑客的两位作者。

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

如果setEnv(..)方法被调用多次，当键从theEnvironment映射中检索时，它们现在是字符串(已被setEnv(...)第一次调用作为字符串放入)，并且不能强制转换为映射的泛型类型Variable,，它是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:

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);
}
}

在Android上，界面是通过Libcore公开的。os作为一种隐藏的API。

Libcore.os.setenv("VAR", "value", bOverwrite);
Libcore.os.getenv("VAR"));

Libcore类和接口操作系统都是公开的。只有类声明缺失，需要显示给链接器。不需要将类添加到应用程序中，但是如果包含了类也无妨。

package libcore.io;


public final class Libcore {
private Libcore() { }


public static Os os;
}


package libcore.io;


public interface Os {
public String getenv(String name);
public void setenv(String name, String value, boolean overwrite) throws ErrnoException;
}

这是@paul-blair的回答转换成Java的组合，其中包括paul blair指出的一些清理，以及@pushy的代码(由@Edward Campbell和anonymous组成)中似乎存在的一些错误。

我不能强调这段代码只应该在测试中使用，而且非常hack。但是对于在测试中需要环境设置的情况，这正是我所需要的。

这还包括我的一些小改动，允许代码在运行的两个Windows上工作

java version "1.8.0_92"
Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)

和Centos一样

openjdk version "1.8.0_91"
OpenJDK Runtime Environment (build 1.8.0_91-b14)
OpenJDK 64-Bit Server VM (build 25.91-b14, mixed mode)

实现:

/**
* Sets an environment variable FOR THE CURRENT RUN OF THE JVM
* Does not actually modify the system's environment variables,
*  but rather only the copy of the variables that java has taken,
*  and hence should only be used for testing purposes!
* @param key The Name of the variable to set
* @param value The value of the variable to set
*/
@SuppressWarnings("unchecked")
public static <K,V> void setenv(final String key, final String value) {
try {
/// we obtain the actual environment
final Class<?> processEnvironmentClass = Class.forName("java.lang.ProcessEnvironment");
final Field theEnvironmentField = processEnvironmentClass.getDeclaredField("theEnvironment");
final boolean environmentAccessibility = theEnvironmentField.isAccessible();
theEnvironmentField.setAccessible(true);


final Map<K,V> env = (Map<K, V>) theEnvironmentField.get(null);


if (SystemUtils.IS_OS_WINDOWS) {
// This is all that is needed on windows running java jdk 1.8.0_92
if (value == null) {
env.remove(key);
} else {
env.put((K) key, (V) value);
}
} else {
// This is triggered to work on openjdk 1.8.0_91
// The ProcessEnvironment$Variable is the key of the map
final Class<K> variableClass = (Class<K>) Class.forName("java.lang.ProcessEnvironment$Variable");
final Method convertToVariable = variableClass.getMethod("valueOf", String.class);
final boolean conversionVariableAccessibility = convertToVariable.isAccessible();
convertToVariable.setAccessible(true);


// The ProcessEnvironment$Value is the value fo the map
final Class<V> valueClass = (Class<V>) Class.forName("java.lang.ProcessEnvironment$Value");
final Method convertToValue = valueClass.getMethod("valueOf", String.class);
final boolean conversionValueAccessibility = convertToValue.isAccessible();
convertToValue.setAccessible(true);


if (value == null) {
env.remove(convertToVariable.invoke(null, key));
} else {
// we place the new value inside the map after conversion so as to
// avoid class cast exceptions when rerunning this code
env.put((K) convertToVariable.invoke(null, key), (V) convertToValue.invoke(null, value));


// reset accessibility to what they were
convertToValue.setAccessible(conversionValueAccessibility);
convertToVariable.setAccessible(conversionVariableAccessibility);
}
}
// reset environment accessibility
theEnvironmentField.setAccessible(environmentAccessibility);


// we apply the same to the case insensitive environment
final Field theCaseInsensitiveEnvironmentField = processEnvironmentClass.getDeclaredField("theCaseInsensitiveEnvironment");
final boolean insensitiveAccessibility = theCaseInsensitiveEnvironmentField.isAccessible();
theCaseInsensitiveEnvironmentField.setAccessible(true);
// Not entirely sure if this needs to be casted to ProcessEnvironment$Variable and $Value as well
final Map<String, String> cienv = (Map<String, String>) theCaseInsensitiveEnvironmentField.get(null);
if (value == null) {
// remove if null
cienv.remove(key);
} else {
cienv.put(key, value);
}
theCaseInsensitiveEnvironmentField.setAccessible(insensitiveAccessibility);
} catch (final ClassNotFoundException | NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
throw new IllegalStateException("Failed setting environment variable <"+key+"> to <"+value+">", e);
} catch (final NoSuchFieldException e) {
// we could not find theEnvironment
final Map<String, String> env = System.getenv();
Stream.of(Collections.class.getDeclaredClasses())
// obtain the declared classes of type $UnmodifiableMap
.filter(c1 -> "java.util.Collections$UnmodifiableMap".equals(c1.getName()))
.map(c1 -> {
try {
return c1.getDeclaredField("m");
} catch (final NoSuchFieldException e1) {
throw new IllegalStateException("Failed setting environment variable <"+key+"> to <"+value+"> when locating in-class memory map of environment", e1);
}
})
.forEach(field -> {
try {
final boolean fieldAccessibility = field.isAccessible();
field.setAccessible(true);
// we obtain the environment
final Map<String, String> map = (Map<String, String>) field.get(env);
if (value == null) {
// remove if null
map.remove(key);
} else {
map.put(key, value);
}
// reset accessibility
field.setAccessible(fieldAccessibility);
} catch (final ConcurrentModificationException e1) {
// This may happen if we keep backups of the environment before calling this method
// as the map that we kept as a backup may be picked up inside this block.
// So we simply skip this attempt and continue adjusting the other maps
// To avoid this one should always keep individual keys/value backups not the entire map
LOGGER.info("Attempted to modify source map: "+field.getDeclaringClass()+"#"+field.getName(), e1);
} catch (final IllegalAccessException e1) {
throw new IllegalStateException("Failed setting environment variable <"+key+"> to <"+value+">. Unable to access field!", e1);
}
});
}
LOGGER.info("Set environment variable <"+key+"> to <"+value+">. Sanity Check: "+System.getenv(key));
}

设置单个环境变量(基于Edward Campbell的回答):

public static void setEnv(String key, String value) {
try {
Map<String, String> env = System.getenv();
Class<?> cl = env.getClass();
Field field = cl.getDeclaredField("m");
field.setAccessible(true);
Map<String, String> writableEnv = (Map<String, String>) field.get(env);
writableEnv.put(key, value);
} catch (Exception e) {
throw new IllegalStateException("Failed to set environment variable", e);
}
}

用法:

首先，把这个方法放在任何你想要的类中，例如SystemUtil。然后静态调用它:

SystemUtil.setEnv("SHELL", "/bin/bash");

如果在此之后调用System.getenv("SHELL")，将返回"/bin/bash"。

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。

我认为大多数困惑的来源来自@anonymous在“theEnvironment”字段上操作的解决方案。查看ProcessEnvironment结构的定义，'theEnvironment'不是一个Map<字符串>但它是一个Map<变量，值>。清除映射工作正常，但是putAll操作重新构建映射(Map<String >，当后续操作使用期望Map<的常规API对数据结构进行操作时，可能会导致问题;变量，值>。此外，访问/删除单个元素也是一个问题。解决方案是通过“不可修改的环境”间接地访问“环境”。但由于这是一个UnmodifiableMap类型，访问必须通过UnmodifiableMap类型的私有变量'm'来完成。参见下面代码中的getModifiableEnvironmentMap2。

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);
}

Kotlin实现我最近基于Edward的回答:

fun setEnv(newEnv: Map<String, String>) {
val unmodifiableMapClass = Collections.unmodifiableMap<Any, Any>(mapOf()).javaClass
with(unmodifiableMapClass.getDeclaredField("m")) {
isAccessible = true
@Suppress("UNCHECKED_CAST")
get(System.getenv()) as MutableMap<String, String>
}.apply {
clear()
putAll(newEnv)
}
}

这是Kotlin邪恶版本的@pushy的邪恶回答 =)

@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。

如果你使用SpringBoot，你可以在下面的属性中添加指定环境变量:

was.app.config.properties.toSystemProperties

基于@pushy的答案的jython变体，适用于windows。

def set_env(newenv):
from java.lang import Class
process_environment = Class.forName("java.lang.ProcessEnvironment")
environment_field =  process_environment.getDeclaredField("theEnvironment")
environment_field.setAccessible(True)
env = environment_field.get(None)
env.putAll(newenv)
invariant_environment_field = process_environment.getDeclaredField("theCaseInsensitiveEnvironment");
invariant_environment_field.setAccessible(True)
invevn = invariant_environment_field.get(None)
invevn.putAll(newenv)

用法:

old_environ = dict(os.environ)
old_environ['EPM_ORACLE_HOME'] = r"E:\Oracle\Middleware\EPMSystem11R1"
set_env(old_environ)

蒂姆·瑞安的回答对我很管用……但我希望它用于Groovy(例如Spock上下文)，并且simplissimo:

import java.lang.reflect.Field


def getModifiableEnvironmentMap() {
def unmodifiableEnv = System.getenv()
Class cl = unmodifiableEnv.getClass()
Field field = cl.getDeclaredField("m")
field.accessible = true
field.get(unmodifiableEnv)
}


def clearEnvironmentVars( def keys ) {
def savedVals = [:]
keys.each{ key ->
String val = modifiableEnvironmentMap.remove(key)
// thinking about it, I'm not sure why we need this test for null
// but haven't yet done any experiments
if( val != null ) {
savedVals.put( key, val )
}
}
savedVals
}


def setEnvironmentVars(Map varMap) {
modifiableEnvironmentMap.putAll(varMap)
}


// pretend existing Env Var doesn't exist
def PATHVal1 = System.env.PATH
println "PATH val1 |$PATHVal1|"
String[] keys = ["PATH", "key2", "key3"]
def savedVars = clearEnvironmentVars(keys)
def PATHVal2 = System.env.PATH
println "PATH val2 |$PATHVal2|"


// return to reality
setEnvironmentVars(savedVars)
def PATHVal3 = System.env.PATH
println "PATH val3 |$PATHVal3|"
println "System.env |$System.env|"


// pretend a non-existent Env Var exists
setEnvironmentVars( [ 'key4' : 'key4Val' ])
println "key4 val |$System.env.key4|"

Kotlin中的一个版本，在这个算法中，我创建了一个装饰器，允许您从环境中设置和获取变量。

import java.util.Collections
import kotlin.reflect.KProperty
​
class EnvironmentDelegate {
operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
return System.getenv(property.name) ?: "-"
}
​
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
val key = property.name
​
val classes: Array<Class<*>> = Collections::class.java.declaredClasses
val env = System.getenv()
​
val cl = classes.first { "java.util.Collections\$UnmodifiableMap" == it.name }
​
val field = cl.getDeclaredField("m")
field.isAccessible = true
val obj = field[env]
val map = obj as MutableMap<String, String>
map.putAll(mapOf(key to value))
}
}
​
class KnownProperties {
var JAVA_HOME: String by EnvironmentDelegate()
var sample: String by EnvironmentDelegate()
}
​
fun main() {
val knowProps = KnownProperties()
knowProps.sample = "2"
​
println("Java Home: ${knowProps.JAVA_HOME}")
println("Sample: ${knowProps.sample}")
}

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相同的内容 这是我使用的(假设- JAVA_LOC是env。变量名)< / p >

        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/程序的生命周期无关

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，允许你做如下事情:

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。

只需使用reflect，如setEnv("k1","v1")

    private void setEnv(String key, String val) throws Exception {
getModifiableEnv().put(key, val);
}
    

private Map<String, String> getModifiableEnv() throws Exception {
Map<String, String> unmodifiableEnv = System.getenv();
Field field = unmodifiableEnv.getClass().getDeclaredField("m");
field.setAccessible(true);
return (Map<String, String>) field.get(unmodifiableEnv);
}

需要

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

在当前Java进程中设置环境变量，该进程使用依赖于所讨论的环境变量的本机代码(dll)，只有在本机设置此环境变量时才有效。

这里的大多数示例都是在JVM中更改映射，但不能在本机工作。

private static void setEnv(String key, String value) {
if(isWindows()) {
if (!Kernel32.INSTANCE.SetEnvironmentVariable(key, value)) {
System.err.println("Unable to set the environemnt variable: " + key);
}
}
}

对于类似unix的操作系统，可以使用LibCAPI接口，但还没有尝试过。

如果你像我一样在测试中面临这个问题，并且你正在使用Junit5， Junit-pioneer附带了非常有用的注释。maven版本

例子:

@Test
@SetEnvironmentVariable(key = "some variable",value = "new value")
void test() {
assertThat(System.getenv("some variable")).
isEqualTo("new value");
}

怎么推荐都不为过。