這個原因應該是沒有在manifest中指定Main-Class吧，要不乖乖寫個bat腳本調用jar指令指定，要不gradle建置時用jar指定

jar {
    from {
        configurations.compile.collect {
            it.isDirectory() ? it : zipTree(it)
        }
    }
    exclude 'META-INF/MANIFEST.MF'
    exclude 'META-INF/LICENSE'
    exclude 'META-INF/*.SF'
    exclude 'META-INF/*.DSA'
    exclude 'META-INF/*.RSA'

    manifest {
        attributes "Implementation-Title": "..."
        attributes "Implementation-Version": version
        attributes "Main-Class": "..."
    }
  
}

使用gradle中的jar任務打包