unit MainUnit;

interface

uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls, Registry;

type

  TfrmMain = class(TForm)

    btn1: TButton;

    edt1: TEdit;

    lbl1: TLabel;

    procedure btn1Click(Sender: TObject);

  private

    { Private declarations }

  public

    { Public declarations }

  end;

var

  frmMain: TfrmMain;

  function ExpandEnvironment(const strValue: string): string;

  function SetGlobalEnvironment(const envName, envValue: string; const isUser: Boolean = True): Boolean;

implementation

{$R *.dfm}

procedure TfrmMain.btn1Click(Sender: TObject);

var

  path:string;

begin

  SetGlobalEnvironment('JAVA_HOME',edt1.Text);

  SetGlobalEnvironment('CLASSPATH','.;%JAVA_HOME%/jre/lib/rt.jar;%JAVA_HOME%/lib/tools.jar;%JAVA_HOME%/lib;');

  path := ExpandEnvironment('%PATH%');

  path := path + ';%JAVA_HOME%/bin';

  SetGlobalEnvironment('PATH',path);

end;

///

///  设置系统级环境变量参数函数

///

/// 环境变量名

/// 环境变量值

/// 为true表示只更新当前操作员的环境变量

///

function SetGlobalEnvironment(const envName, envValue: string; const isUser: Boolean = True): Boolean;

resourcestring

  REG_MLOCATION ='System/CurrentControlSet/Control/Session Manager/Environment';  // 环境变量存储在注册表中的位置

  REG_ULOCATION = 'Environment';

begin

  with TRegistry.Create do

    try

      if isUser then // 设置当前用户的环境变量

        Result := OpenKey(REG_ULOCATION, True)

      else // 设置系统环境变量,对所有用户有效

      begin

        RootKey := HKEY_LOCAL_MACHINE;

        Result := OpenKey(REG_MLOCATION, True);

      end;

      if Result then

      begin

        WriteString(envName, envValue); // 设置注册表中环境变量的值

        SetEnvironmentVariable(PChar(envName), PChar(envValue)); // 更新当前进程的环境变量值

        SendMessage(HWND_BROADCAST, WM_SETTINGCHANGE, 0, Integer(PChar('Environment')));

        // 发送消息通知所有上层的窗口,环境参数发生变化

      end;

    finally

      Free;

    end;

end;

///

///  获取环境变量

///

/// 环境变量名

/// 环境变量值

function ExpandEnvironment(const strValue: string): string;

var

  chrResult: array[0..1023] of Char;

  wrdReturn: DWORD;

begin

  wrdReturn := ExpandEnvironmentStrings(PChar(strValue), chrResult, 1024);

  if wrdReturn = 0 then

    Result := strValue

  else

  begin

    Result := Trim(chrResult);

  end;

end;

end.

来源：https://www.cnblogs.com/toosuo/p/3698587.html