JDK18（或者一些版本都无法支持Applet类）

所以我把jdk改成了这个1.8版本就可以了：
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idea配置

将java3D目录下面的bin目录中的dll文件拷贝到idea里配置的jdk路径的bin目录下：
idea里配置的jdk路径的bin目录：
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把这个文件拷贝过去：

导入java3D的jar包

打开idea，选取环境，创建项目
右键External Libraries下的环境，打开库设置：

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导入这个路径下的jar包：

C:\Program Files\Java\Java3D\1.5.1\lib\ext

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步骤：
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测试代码：

import com.sun.j3d.utils.applet.MainFrame;
import com.sun.j3d.utils.behaviors.mouse.MouseRotate;
import com.sun.j3d.utils.behaviors.mouse.MouseTranslate;
import com.sun.j3d.utils.behaviors.mouse.MouseZoom;
import com.sun.j3d.utils.geometry.Box;
import com.sun.j3d.utils.geometry.Sphere;
import com.sun.j3d.utils.universe.SimpleUniverse;

import javax.media.j3d.*;
import javax.vecmath.Color3f;
import javax.vecmath.Point3d;
import javax.vecmath.Vector3f;
import java.applet.Applet;
import java.awt.*;

public class twoprimitivedisplay extends Applet {
    public twoprimitivedisplay() {//设置显示界面的相关参数
        setLayout(new BorderLayout());
//创建投影平面Canvas3D
        GraphicsConfiguration gc = SimpleUniverse.getPreferredConfiguration();
        Canvas3D c = new Canvas3D(gc);
//将投影平面上的图象显示在显示平面的中间
        add("Center", c);
//设置SimpleUniverse，由系统选择视点在z轴的正向，观察方向沿z轴反向
        BranchGroup BranchGroupScene = createBranchGroup();
        SimpleUniverse u = new SimpleUniverse(c);
        u.getViewingPlatform().setNominalViewingTransform();
//将BranchGroup：BranchGroupScene加入到SimpleUniverse：u中
        u.addBranchGraph(BranchGroupScene);
    }

    public static void main(String[] args) {//通过MainFrame显示图象
        new MainFrame(new twoprimitivedisplay(), 300, 300);
    }

    public BranchGroup createBranchGroup() {//定义BranchGroup
        BranchGroup BranchGroupRoot = new BranchGroup();
//创建球心在坐标系原点球形范围
        BoundingSphere bounds = new BoundingSphere(new Point3d(0.0, 0.0, 0.0), 100.0);
//定义背景颜色
        Color3f bgColor = new Color3f(1.0f, 1.0f, 1.0f);
        Background bg = new Background(bgColor);
        bg.setApplicationBounds(bounds);
        BranchGroupRoot.addChild(bg);
//定义平行光、颜色、照射方向与作用范围
        Color3f directionalColor = new Color3f(1.f, 1.f, 1.f);
        Vector3f vec = new Vector3f(-1.f, -1.f, -1.0f);
        DirectionalLight directionalLight = new DirectionalLight(directionalColor, vec);
        directionalLight.setInfluencingBounds(bounds);
        BranchGroupRoot.addChild(directionalLight);
//定义两个三维型体的外观
        Appearance app1 = new Appearance();
        Material material1 = new Material();
        material1.setDiffuseColor(new Color3f(1.0f, .0f, 0.0f));
        app1.setMaterial(material1);
        Appearance app2 = new Appearance();
        Material material2 = new Material();
        material2.setDiffuseColor(new Color3f(.0f, 1.0f, 0.0f));
        app2.setMaterial(material2);
//定义总的TransformGroup：transformgroup
        TransformGroup transformgroup = new TransformGroup();
//设置对该TransformGroup的读写能力
        transformgroup.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
        transformgroup.setCapability(TransformGroup.ALLOW_TRANSFORM_READ);
//将该TransformGroup加入到BranchGroupRoot中
        BranchGroupRoot.addChild(transformgroup);
//定义鼠标对场景的旋转、平移与放大功能
        MouseRotate mouserotate = new MouseRotate();
        mouserotate.setTransformGroup(transformgroup);
        BranchGroupRoot.addChild(mouserotate);
        mouserotate.setSchedulingBounds(bounds);
        MouseZoom mousezoom = new MouseZoom();
        mousezoom.setTransformGroup(transformgroup);
        BranchGroupRoot.addChild(mousezoom);
        mousezoom.setSchedulingBounds(bounds);
        MouseTranslate mousetranslate = new MouseTranslate();
        mousetranslate.setTransformGroup(transformgroup);
        BranchGroupRoot.addChild(mousetranslate);
        mousetranslate.setSchedulingBounds(bounds);
        /*定义一个球体与一个长方体的大小、外观属性与坐标变换，并定义相应的TransformGroup：tg1、tg2*/
        TransformGroup tg1 = new TransformGroup();
        tg1.addChild(new Sphere(0.4f, app1));
        Transform3D t = new Transform3D();
        t.setTranslation(new Vector3f(0.f, -0.425f, 0.f));
        TransformGroup tg2 = new TransformGroup(t);
        tg2.addChild(new Box(0.5f, 0.05f, 0.5f, app2));
//将定义好的两个TransformGroup(tg1、tg2)加入到总的transformgroup
        transformgroup.addChild(tg1);
        transformgroup.addChild(tg2);
//对BranchGroupRoot预编译
        BranchGroupRoot.compile();
//通过方法名返回BranchGroupRoot
        return BranchGroupRoot;
    }
}