I have a php site with location /app served by Java app server on same server port 9001

What is in /app/ should become / (root) for Java app server

I have this in config:

server {
...
index index.html index.htm index.php;

location /app {

proxy_pass http://localhost:9001;
proxy_next_upstream error timeout invalid_header http_500 http_502 http_503;

proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

proxy_set_header X-Forwarded-Proto https;

proxy_redirect off;
}

location / {
# First attempt to serve request as file, then
# as directory, then fall back to index.html
try_files $uri $uri/ /index.php?$args;
}

location ~ index.php$ {

try_files $uri =404;

include fastcgi_params;
fastcgi_index index.php;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_pass 127.0.0.1:9000;
}
}

with this configuration, Java server receives requests like "GET /app/" while it should receive "GET /"

If I add trailing slash to proxy_pass:

proxy_pass http://localhost:9001/;

Then java server receives "GET //" and unable to serve that either.

Do I need to involve rewrite here ? And how? or extra trailing slash is caused by other rewrite rules somehow?

Thank you!